[MacPorts] WorkingWithGit modified

[Ryan Schmidt]

Thanks everyone for your work in creating this documentation. 

Something I did not see discussed on this page was that after you fork, and before you start making changes, you should create a branch specific to the work you are doing. For example if you are updating port foo to a new version, make a branch called "update-foo". If you later want to do other unrelated work, you create a new branch from master for that. The github pull request interface makes it easy to delete your branch after it's been merged, which only works if you made a branch to begin with.