Added README for doxygen

Author: Ithanil

doc/doxygen/README.md

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+# Doxygen
+
+Doxygen is documentation generator and is able to generate visualization of the code structure.
+
+## Usage
+
+After installing doxygen, invoke the following on the command prompt (inside doxygen folder):
+
+doxygen doxygen.conf
+
+Now you may view the created webpage by opening the doxygen.html symlink with a webbrowser of your choice, e.g.:
+
+firefox doxygen.html