link to test as examples from usage text

as much as possible.  Some tests though are a bit complicated, and may only confuse the user further, so not linking for those commands (stats, fixlengths, slice, select)

src/cmd/dedup.rs

@@ -12,6 +12,8 @@ Either way, the output will not only be deduplicated, it will also be sorted.
 
 A duplicate count will also be sent to <stderr>.
 
+For examples, see https://github.com/jqnatividad/qsv/blob/master/tests/test_dedup.rs.
+
 Usage:
  qsv dedup [options] [<input>]
  qsv dedup --help

src/cmd/exclude.rs

@@ -13,6 +13,8 @@ separating them with a comma. Specify a range of columns with `-`. Both
 columns1 and columns2 must specify exactly the same number of columns.
 (See 'qsv select --help' for the full syntax.)
 
+For examples, see https://github.com/jqnatividad/qsv/blob/master/tests/test_exclude.rs.
+
 Usage:
  qsv exclude [options] <columns1> <input1> <columns2> <input2>
  qsv exclude --help

src/cmd/fill.rs

@@ -29,6 +29,8 @@ CSV is not sorted by the `--groupby` columns, rows may be
 re-ordered during output due to the buffering of rows
 collected before the first valid value.
 
+For examples, see https://github.com/jqnatividad/qsv/blob/master/tests/test_fill.rs.
+
 Usage:
  qsv fill [options] [--] <selection> [<input>]
  qsv fill --help

src/cmd/flatten.rs

@@ -7,6 +7,8 @@ can be changed with the --separator flag.
 There is also a condensed view (-c or --condense) that will shorten the
 contents of each field to provide a summary view.
 
+For examples, see https://github.com/jqnatividad/qsv/blob/master/tests/test_flatten.rs.
+
 Usage:
  qsv flatten [options] [<input>]
  qsv flatten --help

src/cmd/fmt.rs

@@ -7,6 +7,8 @@ pipe multiple qsv commands together. However, you may want the final result to
 have a specific delimiter or record separator, and this is where 'qsv fmt' is
 useful.
 
+For examples, see https://github.com/jqnatividad/qsv/blob/master/tests/test_fmt.rs.
+
 Usage:
  qsv fmt [options] [<input>]
  qsv fmt --help

src/cmd/frequency.rs

@@ -12,6 +12,8 @@ respectively.
 Since this computes an exact frequency table, memory proportional to the
 cardinality of each column is required.
 
+For examples, see https://github.com/jqnatividad/qsv/blob/master/tests/test_frequency.rs.
+
 Usage:
  qsv frequency [options] [<input>]
  qsv frequency --help

src/cmd/headers.rs

@@ -7,6 +7,8 @@ CSV data.
 Note that multiple CSV files may be given to this command. This is useful with
 the --intersect flag.
 
+For examples, see https://github.com/jqnatividad/qsv/blob/master/tests/test_headers.rs.
+
 Usage:
  qsv headers [options] [<input>...]
  qsv headers --help

src/cmd/input.rs

@@ -15,6 +15,8 @@ invalid UTF-8 sequences with �.
 This command is typically used in the beginning of a data pipeline to normalize & prepare CSVs
 for further processing with other qsv commands.
 
+For examples, see https://github.com/jqnatividad/qsv/blob/master/tests/test_input.rs.
+
 Usage:
  qsv input [options] [<input>]
  qsv input --help

src/cmd/jsonl.rs

@@ -8,6 +8,8 @@ fields from the input.
 Also, it will fail if the JSON documents are not consistent with one another,
 as the first JSON line will be use to infer the headers of the CSV output.
 
+For examples, see https://github.com/jqnatividad/qsv/blob/master/tests/test_jsonl.rs.
+
 Usage:
  qsv jsonl [options] [<input>]
  qsv jsonl --help

src/cmd/partition.rs

@@ -4,6 +4,8 @@ Partitions the given CSV data into chunks based on the value of a column
 The files are written to the output directory with filenames based on the
 values in the partition column and the `--filename` flag.
 
+For examples, see https://github.com/jqnatividad/qsv/blob/master/tests/test_partition.rs.
+
 Usage:
  qsv partition [options] <column> <outdir> [<input>]
  qsv partition --help

src/cmd/pseudo.rs

@@ -2,6 +2,8 @@ static USAGE: &str = r#"
 Pseudonymise the value of the given column by replacing them by an
 incremental identifier.
 
+For examples, see https://github.com/jqnatividad/qsv/blob/master/tests/test_pseudo.rs.
+
 Usage:
  qsv pseudo [options] <column> [<input>]
  qsv pseudo --help

src/cmd/python.rs

@@ -75,7 +75,7 @@ Some usage examples:
  With "py map", if a python expression is invalid, "<ERROR>" is returned.
  With "py filter", if a python expression is invalid, no filtering is done.
 
- For more extensive examples, see https://github.com/jqnatividad/qsv/blob/master/tests/test_py.rs.
+For more extensive examples, see https://github.com/jqnatividad/qsv/blob/master/tests/test_py.rs.
 
 Usage:
  qsv py map [options] -n <script> [<input>]

src/cmd/rename.rs

@@ -10,6 +10,8 @@ all of the headers, and separate them by a comma.
  Use column names that contains commas and conflict with the separator:
  $ qsv rename '"Date - Opening","Date - Actual Closing"'
 
+For more examples, see https://github.com/jqnatividad/qsv/blob/master/tests/test_rename.rs.
+
 Usage:
  qsv rename [options] [--] <headers> [<input>]
  qsv rename --help

src/cmd/safenames.rs

@@ -12,6 +12,8 @@ Returns exitcode 1 when no headers are modified.
  Change the name of the columns:
  $ qsv safenames data.csv
 
+For more examples, see https://github.com/jqnatividad/qsv/blob/master/tests/test_safenames.rs.
+
 Usage:
  qsv safenames [options] [<input>]
  qsv safenames --help

src/cmd/sample.rs

@@ -16,6 +16,8 @@ once, which is necessary to provide a uniform random sample. If you wish to
 limit the number of records visited, use the 'qsv slice' command to pipe into
 'qsv sample'.
 
+For examples, see https://github.com/jqnatividad/qsv/blob/master/tests/test_sample.rs.
+
 Usage:
  qsv sample [options] <sample-size> [<input>]
  qsv sample --help

src/cmd/schema.rs

@@ -14,6 +14,8 @@ a `stdin.csv` file will created with stdin's contents as well.
 
 Note that `stdin.csv` will be overwritten if it already exists.
 
+For examples, see https://github.com/jqnatividad/qsv/blob/master/tests/test_schema.rs.
+
 Usage:
  qsv schema [options] [<input>]
  qsv schema --help

src/cmd/sniff.rs

@@ -9,6 +9,8 @@ to adjust).
 If you want more robust, guaranteed schemata, use the "schema" or "stats" commands
 instead as they scan the entire file.
 
+For examples, see https://github.com/jqnatividad/qsv/blob/master/tests/test_sniff.rs.
+
 Usage:
  qsv sniff [options] [<input>]
  qsv sniff --help

src/cmd/tojsonl.rs

@@ -2,7 +2,9 @@ static USAGE: &str = r#"
 Smartly converts CSV to a newline-delimited JSON (JSONL/NDJSON).
 
 By scanning the CSV first, it "smartly" infers the appropriate JSON data type
-for each column. 
+for each column.
+
+For examples, see https://github.com/jqnatividad/qsv/blob/master/tests/test_tojsonl.rs.
 
 Usage:
  qsv tojsonl [options] [<input>]