doc: Added instructions for how to add an upsteam to forked repo
doc: Added instructions for how to add an upsteam to forked repo

As a first time git developer, I struggled to understand whether to create a new fork for each pull request or not.

After asking the IRC chat, I have added this to the documentation to further help new developers using git.

Co-Authored-By: Michael <fanquake@gmail.com>

Former-commit-id: 2564254ea746ada154322139e924a744cb518b7d
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin