doc: Add README instructions for creating private git fork

[rg0now]

Describe how to:

• create a private git fork using the command line (cannot fork with github as it refuses to create a private fork from a public repo)
• fork the private repo locally or set set the private repo as the default git remote if the local working copy has already been forked from the main repo.

[levaitamas]

this gist might be useful: https://gist.github.com/0xjac/85097472043b697ab57ba1b1c7530274