Mention virtualenv'ing in hacking instructions

README.rst

@@ -51,11 +51,13 @@ To install the latest development version::
 
   pip install setuptools 'cython>=0.23.4' git+git://github.com/gevent/gevent.git#egg=gevent
 
-To hack on gevent::
+To hack on gevent (using a virtualenv)::
 
   $ git clone https://github.com/gevent/gevent.git
   $ cd gevent
-  $ pip install -r dev-requirements.txt
+  $ virtualenv env
+  $ source env/bin/activate
+  (env) $ pip install -r dev-requirements.txt
 
 .. note::
    You must have Cython, a C compiler, and the Python

dev-requirements.txt

+setuptools
 cython>=0.23.4
 greenlet>=0.4.9
 coverage>=4.0
 cffi
-setuptools
 -e .