Add instructions for upgrading setuptools for when people encounter a message...

Add instructions for upgrading setuptools for when people encounter a message "mock requires setuptools>=17.1"

README.rst

@@ -121,6 +121,7 @@ To install the synapse homeserver run::
 
     virtualenv -p python2.7 ~/.synapse
     source ~/.synapse/bin/activate
+    pip install --upgrade setuptools
     pip install --process-dependency-links https://github.com/matrix-org/synapse/tarball/master
 
 This installs synapse, along with the libraries it uses, into a virtual
@@ -285,6 +286,11 @@ may need to manually upgrade it::
 
     sudo pip install --upgrade pip
 
+Installing may fail with ``mock requires setuptools>=17.1. Aborting installation``.
+You can fix this by upgrading setuptools::
+
+    pip install --upgrade setuptools
+
 If pip crashes mid-installation for reason (e.g. lost terminal), pip may
 refuse to run until you remove the temporary installation directory it
 created. To reset the installation::