Add documentation for tags command

docs/commands/tags.md

@@ -0,0 +1,47 @@
+# tags
+
+The tags commands allows users to access the metadata of the previous value in
+the pipeline. This command may be run on multiple values of input as well.
+
+As of writing this, the only metadata returned includes:
+
+- `span`: the start and end indices of the previous value's substring location
+- `anchor`: the source where data was loaded from; this may not appear if the
+ previous pipeline value didn't actually have a source (like trying to `open` a
+ dir, or running `ls` on a dir)
+
+## Examples
+
+```shell
+> open README.md | tags
+━━━━━━━━━━━━━━━━┯━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+ span │ anchor
+────────────────┼──────────────────────────────────────────────────
+ [table: 1 row] │ /Users/danielh/Projects/github/nushell/README.md
+━━━━━━━━━━━━━━━━┷━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+```
+
+```shell
+> open README.md | tags | get span
+━━━━━━━┯━━━━━
+ start │ end
+───────┼─────
+ 5 │ 14
+━━━━━━━┷━━━━━
+```
+
+```shell
+> ls | tags | first 3 | get span
+━━━┯━━━━━━━┯━━━━━
+ # │ start │ end
+───┼───────┼─────
+ 0 │ 0 │ 2
+ 1 │ 0 │ 2
+ 2 │ 0 │ 2
+━━━┷━━━━━━━┷━━━━━
+```
+
+## Reference
+
+More useful information on the `tags` command can be found by referencing [The
+Nu Book's entry on Metadata](https://book.nushell.sh/en/metadata)