readme: declare minimum PyPy version in README
@@ -21,8 +21,8 @@ Requirements
============
This extension is designed to run with Python 2.7, 3.4, 3.5, 3.6, and 3.7
-on common platforms (Linux, Windows, and OS X). x86 and x86_64 are well-tested
-on Windows. Only x86_64 is well-tested on Linux and macOS.
+on common platforms (Linux, Windows, and OS X). On PyPy (both PyPy2 and PyPy3) we support version 6.0.0 and above.
+x86 and x86_64 are well-tested on Windows. Only x86_64 is well-tested on Linux and macOS.
Installing
==========
