Update contributing docs with current practices

[mgrauer]

Based on some discussions at the 2019 Hackathon at Janelia, we want to update the contributing documentation to reflect the evolving development practices.

Points to update:

• Running tests and flake8 locally, satisfy them before pushing PRs
• Squashing commit messages
  • Removing trivial changes (running flake8 locally will help)
  • Having a cleaner git history
  • Pitfalls with QA when squashing (lost references)
  • Get help from core developers if unsure
• Adding a test to exercise a bug you don't know how to fix
  • Use github drafts
  • Issue where would fail tests if merged, so wait until fix