"Edit on GitHub" button for the website

Not sure when I'll find time to work on this but it's a cool feature for the website - so if someone else wants to beat me to it please do!

On the ASF builds mailing list, the Struts team has mentioned their "Edit on GitHub" button, like at the top of http://struts.apache.org/getting-started/

In our case we need a link like https://github.com/apache/sling-site/edit/master/src/main/jbake/content/index.md with the path after /content computed from the actual content path.

The cool thing is that GitHub either edits the page directly, if the logged in user is allowed to do that, or suggest forking if not.