DOC: Community guidelines

[jsheunis]

Ideally, the software should contain guidelines for contributors to allow them to contribute productively to the code, docs or any other aspect of the package. This would include info such as:

• how/where to ask questions
• what process to follow to contribute code
• how to run tests
• anything else worth knowing when contributing

This usually takes the form of a CONTRIBUTING.md document at the root of the repo as well as a section with the same content in the docs.

Ping https://github.com/openjournals/joss-reviews/issues/5706

[LMBooth]

Added contributing.md to root of repository and equivalent in the read the docs.