[README] Missing community guidelines for contribution, issues, and support

[napulen]

This issue is related to the following review: https://github.com/openjournals/joss-reviews/issues/3784

According to the JOSS guidelines, the README should have some community guidelines for the following things:

• [x] Contributing to the library
• [x] Reporting an issue
• [x] Obtaining support

Considering that this is a relatively small single-contributor library, I'd be okay assuming that the third point is not necessary. However, the guidelines for contribution and reporting issues would be very useful.

[MichaelSel]

Added CONTRIBUTING.md to address those issues.