docs: missing CONTRIBUTING.md that was mentioned in README #425

Hi, I'm currently learning get_it from scratch, and I feel this package and codebase is awesome. Thanks for good package:)

BTW, I found that there is a missing hyperlink in README that is pointing CONTRIBUTING.md, which doesn't exist.

I just want to know whether the absence of CONTRIBUTING.md was intentional.
If so, I suggest to remove a line in README which is mentioning CONTRIBUTING.md.

If not, I just hope CONTRIBUTING.md gets back to the codebase.

Thanks for good package again, and if you are busy, I would be happy to prepare a PR for this issue.