spell out that local libs may need to be explicitly given priority

README.rst

@@ -108,6 +108,15 @@ To install the synapse homeserver run::
 This installs synapse, along with the libraries it uses, into
 ``$HOME/.local/lib/`` on Linux or ``$HOME/Library/Python/2.7/lib/`` on OSX.
 
+Your python may not give priority to locally installed libraries over system
+libraries, in which case you must add your local packages to your python path::
+
+    $ # on Linux:
+    $ export PYTHONPATH=$HOME/.local/lib/python2.7/site-packages
+
+    $ # on OSX:
+    $ export PYTHONPATH=$HOME/Library/Python/2.7/lib/python2.7/site-packages
+
 For reliable VoIP calls to be routed via this homeserver, you MUST configure
 a TURN server.  See docs/turn-howto.rst for details.