Helpful pointers for future contributors

Mostly an attempt at pointing people that send pull requests on Github
in the right direction.

CONTRIBUTING

+Contributing to Varnish Cache
+=============================
+
+Varnish Cache is developed with Git hosted on our own infrastructure.
+
+Official development tree is here:
+
+    git://git.varnish-cache.org/varnish-cache
+
+Patches can be sent to varnish-dev@varnish-cache.org.
+
+There is a READ-ONLY git-mirror on Github.  Please do not send contributions
+there, we need them by email to -dev@ for now.
+
+