docs: recommend python3 and virtualenv (apache#901)

* docs: make it clear that python3 is the recommended version

* docs: recommend installing inside a virtualenv

And add a virtualenv primer.

docs/installation.rst

@@ -4,8 +4,8 @@ Installation & Configuration
 Getting Started
 ---------------
 
-Caravel is currently only tested using Python 2.7.*. Python 3 support is
-on the roadmap, Python 2.6 won't be supported.
+Caravel is tested using Python 2.7 and Python 3.4+. Python 3 is the recommended version,
+Python 2.6 won't be supported.
 
 
 OS dependencies
@@ -48,6 +48,26 @@ attempt it, download `get-pip.py <https://bootstrap.pypa.io/get-pip.py>`_, and r
     # You may also have to create C:\Temp
     C:\> md C:\Temp
 
+Python virtualenv
+-----------------
+It is recommended to install Caravel inside a virtualenv. Python 3 already ships virtualenv, for
+Python 2 you need to install it. If it's packaged for your operating systems install it from there
+otherwise you can install from pip: ::
+
+    pip install virtualenv
+
+You can create and activate a virtualenv by: ::
+
+    # virtualenv is shipped in Python 3 as pyvenv
+    virtualenv venv
+    . ./venv/bin/activate
+
+On windows the syntax for activating it is a bit different: ::
+
+    venv\Scripts\activate
+
+Once you activated your virtualenv everything you are doing is confined inside the virtualenv.
+To exit a virtualenv just type ``deactivate``.
 
 Caravel installation and initialization
 ---------------------------------------