Updates to GitHub Cheatsheet

Updated How to Fork and added instructions for syncing your fork with the main project (could be useful for people if any of the core non-content files are updated)
Thanks!! Keeping a repository up to date is one of the things I'm still uncertain about ito how to make it easy for someone who doesn't know git. I have this strange feeling it can be done in the web interface by using pull requests, but it probably amounts to lots of funny steps and black magic :(

We should ask someone with the GitHub desktop to add instructions, I don't think everyone would be brave enough for the command line.