Update README

Author: asottile

README.md

@@ -2,13 +2,18 @@
 [![Coverage Status](https://img.shields.io/coveralls/asottile/setuptools-golang.svg?branch=master)](https://coveralls.io/r/asottile/setuptools-golang)
 
 setuptools-golang
-===================
+=================
 
 A setuptools extension for building cpython extensions written in golang.
 
 ## Requirements
 
-This requires golang >= 1.5 to be installed on your system.
+This requires golang >= 1.5.  It is currently tested against 1.5 and 1.6.
+
+This requires python >= 2.7.  It is currently tested against 2.7, 3.4, 3.5,
+and pypy.
+
+It is incompatible with pypy3 (for now) due to a lack of c-api.
 
 ## Usage