Git workflow added to Developer Manual

[bksahu]

Thank your for contributing to Nuitka!

!! Please check that you select the develop branch (details see below link) !!

Before submitting a PR, please review the guidelines:
Contributing Guidelines

What does this PR do?

Added git workflow to Dev Manual

PR Checklist

• Correct base branch selected? develop for new features and bug fixes too.
  If it's part of a hotfix, it will be moved to master during it.
• All tests still pass. Check the developer manual about Running the Tests.
  There are Travis tests that cover the most important things however, and you
  are welcome to rely on those, but they might not cover enough.
• Ideally new features or fixed regressions ought to be covered via new tests.
• Ideally new or changed features have documentation updates.

[kayhayen]

Thanks a lot @bksahu very helpful.