Update README.md

README.md

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 # open-dis-cpp
 A C++ implementation of the Distributed Interactive Simulation (DIS) protocol
+
+## Git submodule
+This repository includes a git submodule for the XML descripton files. These
+files contain a machine-readable description of the DIS messages that are 
+translated into a language implementation. Since the XML file is shared across
+multiple languages, I'm trying to prevent between-language drift of the description
+file.
+
+You should only need this if you're regenerated the source code, which is unlikely.
+but if you are, bring in the submodule with
+
+~~~~
+git submodule init
+git submodule update
+~~~~