Add bumpversion developer documentation

[clelange]

Clean up the following

Workflow

git checkout -b featurebranch
hack-hack-hack
git add -u
git commit
git push origin featurebranch

--> merge

git checkout master
git pull
bumpversion patch
git push origin master --tags

.bumpversion.cfg contains the list of files that contain the version numbers, e.g. setup.py

patch = + 0.0.1
minor = + 0.1.0
major = + 1