Added doc for libsumo installation on Mac

docs/libsumo_mac.md

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+# 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.