Add to CONTRIBUTING which git workflow is prefered and on what to follow PEP8

Created by: juga0

So far we've being using rebase workflow, but it's not specified in CONTRIBUTING if it's prefered instead of merge. The line We use flake8 to enforce standard python coding style. should be changed to:

• on what we use PEP8 (not all of it is being followed)
• that flake8 is used to check some PEP8 errors