PrettyPageHandler: remove deprecated notices

Until we handle deprecated errors in a graceful manner,
triggering this aborts the application, which defeats the purpose of compatibility.
Also see issue #100.
Contributing: simplify

The forking process is explained imprecisely, and people can figure it
out using GitHub help anyway.

The less text there are the better chances of reading it.
Contributing: add a link to a list of possible features
Contributing: add an advice to ask our opinion before investing much time
PSR-4, HHVM, branch alias