Add doc on contributing during sprints

[agjohnson]

Currently, we don't have documentation on contributing to sprints. The document should cover:

• What can I work on?
  • https://waffle.io/rtfd/readthedocs.org?milestone=Cleanup,Sprints
  • Translations! (also we need better translation docs)
• How to work on tasks:
  • Cleanup tasks are likely to touch all parts of the codebase, try to break up work in to small chunks and communicate what you are working on. Request merges early and often.
• How do i get started?
  • Don't worry about docker/etc for cleanup tasks
  • Link to general setup docs

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[humitos]

Marking this as accepted since we still need to have this document written. Although, it's probably low priority.