Edited README so that the tips and tricks / tutorial are not in the middle of the contribution guide
Want to have an easy, step-by-step contribution guide in the README but wanted to avoid having the git basics also in the middle of the guide. Moved the tips and tricks to follow the contribution guide. These tips and tricks might end up getting moved into some other document.
I also point to one git tutorial webpage so that newcomers who have no git experience can get some guidance for how to use git.