[Documentation] Add contribution file with guidelines

[rickie95]

Part of reviewing process for JOSS acceptance (openjournals/joss-reviews/issues/4681)

I couldn't find clear instructions on how to participate in this project. Ideally you should provide a CONTRIBUTE file with some informations about:

• How to contribute to the software (e.g. how to create a pull request or minimum requirements for accept a PR )
• How to report issues: your procedure of choice for handle bug reports (is there a template to follow? is required to provide a minimum reproducible example? is ok just a simple issue opened on GitHub?)
• How to reach support: alternate channels where to get help (email, forum, ecc..)

[gattia]

Thank you for the comment!

Closing because the following commit adds a CONTRIBUTING.md with extensive documentation on how to contribute and references it in the README.

https://github.com/siavashk/pycpd/commit/08e8cf0b86018948e670cdf1d5e10b450c4547b1