Add MacOS X instructions to doc generation README

doc/README.md

@@ -15,18 +15,28 @@ make clean-all    # remove entire build folder and any cached data
 ## Installing prerequisites
 
 To run the documention build toolchain Python 3 and virtualenv have to be
-installed. The following are instructions for common Linux distributions:
+installed. Here are instructions for common setups:
 
-### virtualenv
-
-#### Ubuntu
+### Ubuntu
 
 ```bash
 sudo apt-get install python-virtualenv
 ```
 
-#### Fedora
+### Fedora
 
 ```
 sudo yum install python-virtualenv
 ```
+
+### MacOS X
+
+## Python 3
+
+Download the latest Python 3 MacOS X package from https://www.python.org and install it.
+This will install both Python 3 and pip3.
+
+## virtualenv
+
+Once Python 3 is installed, open a Terminal and type `pip3 install virtualenv`. This will
+install virtualenv from the Python Package Index.