add a link to the documentation in __init__.py

gevent/__init__.py

@@ -2,6 +2,8 @@
 """
 gevent is a coroutine-based Python networking library that uses greenlet
 to provide a high-level synchronous API on top of libevent event loop.
+
+See http://www.gevent.org/ for the documentation.
 """
 
 version_info = (0, 13, 0)