Add instructions for OSX

[mcneish1]

Is your feature request related to a problem? Please describe.
Current master doesn't ship with executables for Mac, but instructions to create them don't seem to be present.

Describe the solution you'd like
The following steps worked for me, though they may need some adjustment.

1. Grab EPANET 2.1 from https://github.com/OpenWaterAnalytics/EPANET/
2. Edit build/CMake/CMakeLists.txt:19 to say
   add_library(epanet SHARED ${EPANET_SOURCES})
3. cd build/CMake ; cmake . ; make
4. Create a folder called epanet_matlab_toolkit/mac in this project. Copy runepanet and libepanet.dylib from build/CMake/bin here. Copy include/epanet.h here, renamed to libepanet.h
5. Edit epanet_matlab_toolkit/epanet.m in this project to uncomment lines 427-430

[Mariosmsk]

Hi @ mcneish1, thanks for the instructions. We have include mac libraries for dev 2.2 here:
https://github.com/OpenWaterAnalytics/EPANET-Matlab-Toolkit/tree/dev-2.2-edit