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: 2e35f469c4044b97c920106deac6cfb8d4954728
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin2,syscoin/syscoin