Update README.md

Add internal documentation link

README.md

@@ -21,3 +21,6 @@ The **MATSim input files, output files, analysis data and visualizations** are l
 Handling of large files within git is not without problems (git lfs files are not included in the zip download; we have to pay; ...).  In consequence, large files, both on the input and on the output side, reside at https://svn.vsp.tu-berlin.de/repos/public-svn/matsim/scenarios/countries/de/stuttgart .  
 
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+## Internal Documentation
+The internal Documentation is in [GoogleDocs](https://docs.google.com/document/d/1zPxg6FG5F4-x2lRhqHuCPejzcLVhnweKJPqfWUJnFeY/edit#heading=h.el5yhw77k0pg)