Add pre-commit support

[alexandregv]

pre-commit is "A framework for managing and maintaining multi-language pre-commit hooks", so kinda like a local CI. It's a popular tool with a lot of community-made hooks.
I thought it would be very handy to have pre-commit support in goreadme, allowing to rewrite the readme each time the doc is changed, even before pushing.

The PR only consists of a new file used by pre-commit, and the new instructions in README. It has no impact on goreadme itself.
I tested it on a private repo and it works well.

[alexandregv]

I just added a commit making the "Use as a command line tool" sentence a header too, since it was currently included in the "GitHub Action" section, which felt wrong. Let me know if that doesn't seem right to you.
Btw, a little off-topic, but shouldn't it be updated to go install github.com/posener/goreadme/cmd/goreadme@latest ?

[posener]

Sorry for the late reply.

This is cool.

I have no objection to add it.

Is there any way to test the integration?

[alexandregv]

Hi, thanks for the reply!

I just created a demo repo, here: https://github.com/alexandregv/goreadme-precommit-demo
I pushed a main.go file with some sample Go doc, but I voluntarily skipped the pre-commit run, so you can do it yourself.

1. Install pre-commit: https://pre-commit.com/#install
2. Clone the demo repo
3. In the cloned repo, run pre-commit install (to register the hooks in .git/hooks/)
4. Modify the main.go file so you can commit it
5. Commit, see the pre-commit logs
6. Check the modified README ! It should now contain the go doc info I skipped earlier.

You can also force-run pre-commit at all times with pre-commit run --all-files.

[posener]

Looks good.
Thanks for the contribution.