Add note about how to setup the github service hook

The service hook for changes to the wiki has to be configured manually
via the github API. This is because the standard web hook configurable
through settings for the crosswalk-website project does not send
notifications for changes to the wiki.
Invalidate the cache for wiki pages changed on github

The regen.php script will get called by the hook each time a
gollum (wiki) event occurs on the crosswalk-website project wiki.

We now loop through the changed pages notified in the event, and
remove the cached HTML for any which have changed.