Added instructions on how to compile docs for OSX

README.md

@@ -4,6 +4,8 @@ Python port of Anders and Briegel' s [method](https://arxiv.org/abs/quant-ph/050
 ## Documentation
 You can read the full documentation [here](https://peteshadbolt.co.uk/abp/). You can also build it locally using Sphinx with `make doc`.
 
+To install Sphinx on OSX, use `pip install sphinx`. If after doing so `make doc` still does not work, some OSX users may also need to install `sphinxcontrib-napoleon` by running `pip install sphinxcontrib-napoleon`.
+
 ## Installation
 It's easiest to install with `pip`: