Add Conventional Commits to Contributor section of README

[waltjones]

Fixes #860

This does not add commitizen to project dependencies, based on nodejs.org recommendation:

If you’re installing something that you want to use in your program, using require('whatever'), then install it locally, at the root of your project.

If you’re installing something that you want to use in your shell, on the command line or something, install it globally, so that its binaries end up in your PATH environment variable.

https://nodejs.org/en/blog/npm/npm-1-0-global-vs-local-installation/

[ArturMoczulski]

In rollbar-php I added commitizen as a dev dependency. Do you think that's not a good idea?

[waltjones]

I think it has to added globally to use it, and I also needed a ~/.czrc for things to work. So I don't know if there's any advantage to also adding as a dev dependency.

[ArturMoczulski]

Cool. Let me double check how I did it for PHP

[ArturMoczulski]

Ok, I agree with. I will use your README.md guidelines to be used in the PHP SDK as well. Although in the example, I would add the fix #[github_issue] and re #[github_issue] part even though is optional to encourage contributors more to be clear what issue the commit is related to. Possibly the scope as well. Like this:

fix(logging messages): apply logger formatting to Rollbar messages

fix #311

[ArturMoczulski]

Also I would an example in the contrib section how to structure the commit message if there are breaking changes. This is all in the conventional changelog docs, but I'd still add it. What do you think?

[ArturMoczulski]

I made those modifications in the PHP repo: rollbar/rollbar-php#450