[JOSS Review] Community guidelines

[gsquared94]

• Create a CONTRIBUTING.md file (linking to your existing docs/development/ is a good start).
• Update the README.md to direct bug reports to the GitHub Issues tab.

[tjkerby]

Thanks @gsquared94!

Implemented both changes:

• CONTRIBUTING.md: 3f327f4
• README bug report direction → GitHub Issues: fe159f0

Let me know if anything else should be tweaked.