Document how to get the gpiozero version number

docs/notes.rst

@@ -70,3 +70,26 @@ In this case, all references to items within GPIO Zero must be prefixed::
 
     button = gpiozero.Button(2)
 
+How can I tell what version of gpiozero I have installed?
+=========================================================
+
+The gpiozero library relies on the setuptools package for installation
+services.  You can use the setuptools ``pkg_resources`` API to query which
+version of gpiozero is available in your Python environment like so::
+
+    >>> from pkg_resources import require
+    >>> require('gpiozero')
+    [gpiozero 1.2.0 (/usr/local/lib/python2.7/dist-packages)]
+    >>> require('gpiozero')[0].version
+    '1.2.0'
+
+If you have multiple versions installed (e.g. from ``pip`` and ``apt-get``)
+they will not show up in the list returned by the ``require`` method. However,
+the first entry in the list will be the version that ``import gpiozero`` will
+import.
+
+If you receive the error "No module named pkg_resources", you need to install
+the ``pip`` utility. This can be done with the following command in Raspbian::
+
+    $ sudo apt-get install python-pip
+