Improve documentation for --only flag

[gokcelb]

Provide examples about how --only flag works in README.md and flag explanation.

There was no information about what kind of format to pass to the --only flag and this caused some issues with running complexity as expected. The examples provided via this change show the correct format.

[joshuaclayton]

I agree that the documentation is a good first step; I'd also consider adjusting the flag name (perhaps in a separate PR) so it's more clear what the flag is filtering against, and do a minor version bump since it's still pre-1.0.

I'd reviewed rg, which allows filtering based on a --glob flag. This isn't quite what we'd want (it supports regular expressions and is more permissive than what we'd want to start with, I think). fd, on the other hand, does provide something closer: -e/--extension. It can be called as fd -e md -e js for multiple extensions

So, we may want to adjust our standard case only to extension (with the e shorthand). This:

1. improves clarity on what the flag does
2. mirrors another CLI tool that allows filtering by multiple extensions