Added doc for libsumo installation on Mac (#1005)

* Added doc for libsumo installation on Mac

* Added code shell

docs/libsumo_mac.md

@@ -0,0 +1,23 @@
+# How to install Libsumo for Mac OS
+
+This is adapted from an email exchange with the SUMO staff.
+
+
+
+To install libsumo requires re-building and installing SUMO from source.
+
+## Steps
+
+- **Install swig:** ```brew install swig```
+- **Clone the repo:** ```git clone https://github.com/eclipse/sumo.git```
+- **Create a “cmake-build” directory inside sumo/build/ and navigate to it:** ```mkdir build/cmake-build && cd build/cmake-build```
+
+**The next 3 steps are inside that directory**
+
+- ```cmake ../..```
+- ```make```
+- ```make install```
+
+## Additional Notes
+- You can test if libsumo has been built looking at (./testlibsumo) inside the sumo/bin/ directory.
+- Bear in mind to use libsumo with the same Python version with which CMake built SUMO.