Add a wiki page on contributing project pages

A comprehensive guide on submitting project pages using pull requests.
Yes, I think it is easy enough to copy and paste into the new file. You probably have to click the "Raw" button of the example before copying it though.

I would just replace the instructions. To do command line git, you have to setup an SSH key and all sorts of extra steps that I think will turn people off. You could just mention (with links) that there are other ways to do this, like command line git or GitHub for Windows/Mac.