add specification to get started documenting

docs/specification.md

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+# Specification
+
+This specification serves as a starting point to document how `nufmt` should work when formatting nushell scripts. It's meant to be very basic at this point, gradually covering more of the language features.
+
+
+## Supported Features
+
+### Indentation
+
+There should be an `--indent` parameter to allow one to specify the number of spaces to use for indentation.
+
+### Supported Commands
+
+This is the list of the supported commands and their idiomatic formatting.
+
+#### if
+
+The if clause should look like this. Indentation will be covered by the `--indent` flag but for these examples, 2 spaces will be used.
+
+```rust
+if condition {
+ // some thing
+} else if {
+ // some other thing
+} else {
+ // else the last thing
+}
+```