Update contribution guidelines

[AoifeHughes]

The main readme points to https://github.com/alan-turing-institute/rds-course/blob/develop/coursebook/README.md

In this file, we should make it clearer how someone could contribute to the course.

For example:

• Would we welcome PRs from the community?
• Is feedback best given via issues
• What if someone wants to add a section/page/module
  • Or update an existing one?
• How should bugs be reported, do we want to make a template for this?
• How can people flag content problems e.g., https://github.com/alan-turing-institute/rds-course/issues/95

[AoifeHughes]

@callummole started this as part of our large collection of streamlining changes: https://github.com/alan-turing-institute/rds-course/pull/138

However, for these to be complete more work is needed; therefore I have moved this to the appendix branch (branched from streamlining): https://github.com/alan-turing-institute/rds-course/tree/Appendix