"Contributing" page

[lheagy]

We should have a page on how to contribute to the GPG with screen-shots that walk users from the starting point of using the "edit-on-github" through to generating a pull request with a few pointers of what happens in a pr review

cc @dougoldenburg @sgkang. @thast: would you be willing to champion this?

[thast]

For the scope of the current class, my choice would be first to advertise the comment section to the students. We avoid the filter that is Github for most of them.