Add a wiki page on contributing project pages

A comprehensive guide on submitting project pages using pull requests.
You don't actually have to do this on your local computer. You can do it all in the web browser.

Once you fork, you should create a new branch called `gh-pages`. Then you can view the site at `http://<user>.github.io/ev3dev.github.io/projects`.

If you browse to `https://github.com/<user>/ev3dev.github.io/_posts`, the is a button in the web interface to add a new file, which you can edit in the browser.

Screenshots of you actually doing this would be helpful.