ENH: add info on how to add upstream with git

[nathanshammah]

Memo of the protocol to add:

• fork the repo
• clone your fork locally
• add an upstream to your local branch:

git remote add upstream https://github.com/main-org/repo

[nathanshammah]

Actually use

git remote add upstream https://github.com/ORIGINAL_OWNER/ORIGINAL_REPOSITORY.git

see this link for information.

[nathanshammah]

Also add the information, about unsync-ed remote branches (origin behind/above upstream):

git fetch upstream
git checkout master
git reset --hard upstream/master
git push origin master --force

from https://github.com/migueldeicaza/gui.cs/issues/498#issuecomment-632739227