docs: work on feature branch, rebase onto current master

[ThomasWaldmann]

Maybe we could add this to the developer docs.

Sometimes contributors work on new features (or other changes) in their local master branch.
This has some issues, like e.g.:

• not being able to update the local master from upstream master
• not being able to work on multiple different features/changes/fixes

Thus, best practice is to:

• keep master branch just as an always up to date copy of upstream master
• always work in separate feature / fix branches

Initial clean up, if work was already done in master branch:

git checkout master  # make sure this is a clean checkout without any pending stuff
git checkout -b MYFEATURE  # now you have your code in a feature branch
git checkout master
git log  # find the commit hash of the last commit before you began working on your feature
git reset --hard THATHASH
git pull borg master  # borg = upstream repo, updates your local master to the current state there

Now that we have a current master branch, we can also rebase our feature branch onto that:

git checkout MYFEATURE
git rebase -i master  # interactive rebase onto a current master branch
git log  # check: all looking good?
git diff master  # check: all looking good?
# if all looks good:
git push -f  # update your PR on github (force is required here)