doc(changes): inform about new progress API
doc(changes): inform about new progress API

Related to #450

expobrain/GitPython,nvie/GitPython,jeblair/GitPython,nvie/GitPython,breavo/GitPython,gitpython-developers/gitpython,gitpython-developers/GitPython,gitpython-developers/gitpython,breavo/GitPython,jeblair/GitPython,gitpython-developers/GitPython,expobrain/GitPython