README.rst: Update build instructions

This adds virtualenv to the build procedure to make sure that the
documentation gets built with the correct python version.

Closes coala#514

README.rst

@@ -11,7 +11,20 @@ The coala repository can be found
 How to build
 ============
 
-You should run this command before trying to build the documentation:
+Install ``virtualenv`` if you don't have it yet:
+
+::
+
+    pip3 install virtualenvwrapper
+
+Create a new ``virtualenv`` and activate it:
+
+::
+
+    virtualenv -p python3 coala
+    source coala/bin/activate
+
+And then run this command before trying to build the documentation:
 
 ::