Documentation of commit style in PRs

[esheppa]

As we are using the "Rebase and Merge" strategy rather than the "Squash and Merge" strategy to merge PRs, we require fixes to be done in the underlying commits rather than in new commits during PR review. We should document this and have some recommendations of how to achieve this, something along the lines of usage of:

Redoing commits

• git rebase -i HEAD~n and using edit, squash and exec
  Splitting commits
• git reset to set up the state for subsequently using git add -i
• git add -i and using the patch mode to stage some but not all of a given diff for a file

With the understanding that there are many ways to achieve the same result and this is just one of the possible ways