Update installation instructions

Author: nginyc

docs/src/user/client-installation.include.rst

@@ -1,15 +1,10 @@
-1. Install Python 3.6
+1. Install Python 3.6 (`Ubuntu <http://ubuntuhandbook.org/index.php/2017/07/install-python-3-6-1-in-ubuntu-16-04-lts/>`__, `MacOS <https://www.python.org/downloads/mac-osx/>`__)
 
-2. Clone the project at https://github.com/nginyc/rafiki
+2. Clone the project at https://github.com/nginyc/rafiki (e.g. with `Git <https://git-scm.com/downloads>`__)
 
 3. Within the project's root folder, install Rafiki Client's Python dependencies by running:
 
     ::
 
-        pip install -r ./rafiki/client/requirements.txt
+        pip3.6 install -r ./rafiki/client/requirements.txt
 
-4. Set ``$PYTHONPATH`` to the project's root folder:
-
-    ::
-
-        export PYTHONPATH=$PWD