MINIFI-273 Update README to include doxygen dependencies for build.
MINIFI-273 Update README to include doxygen dependencies for build.

This closes #82.

Signed-off-by: Bryan Rosander <ed54063ee8b37e245461007eacb669a92be303f7@apache.org>

dtrodrigues/nifi-minifi-cpp,achristianson/nifi-minifi-cpp,NiFiLocal/nifi-minifi-cpp,benqiu2016/nifi-minifi-cpp,brosander/nifi-minifi-cpp,jdye64/nifi-minifi-cpp,apiri/nifi-minifi-cpp,achristianson/nifi-minifi-cpp,minifirocks/nifi-minifi-cpp,minifirocks/nifi-minifi-cpp,minifirocks/nifi-minifi-cpp,jdye64/nifi-minifi-cpp,achristianson/nifi-minifi-cpp,phrocker/nifi-minifi-cpp,reynoldsjohn/nifi-minifi-cpp,dtrodrigues/nifi-minifi-cpp,benqiu2016/nifi-minifi-cpp,apiri/nifi-minifi-cpp,dtrodrigues/nifi-minifi-cpp,kevdoran/nifi-minifi-cpp,apiri/nifi-minifi-cpp,benqiu2016/nifi-minifi-cpp,brosander/nifi-minifi-cpp,phrocker/nifi-minifi-cpp,dtrodrigues/nifi-minifi-cpp,NiFiLocal/nifi-minifi-cpp,kevdoran/nifi-minifi-cpp,dtrodrigues/nifi-minifi-cpp,kevdoran/nifi-minifi-cpp,NiFiLocal/nifi-minifi-cpp,apiri/nifi-minifi-cpp,jdye64/nifi-minifi-cpp,phrocker/nifi-minifi-cpp,achristianson/nifi-minifi-cpp,reynoldsjohn/nifi-minifi-cpp,minifirocks/nifi-minifi-cpp,NiFiLocal/nifi-minifi-cpp,apiri/nifi-minifi-cpp,brosander/nifi-minifi-cpp,phrocker/nifi-minifi-cpp,reynoldsjohn/nifi-minifi-cpp,jdye64/nifi-minifi-cpp,phrocker/nifi-minifi-cpp,kevdoran/nifi-minifi-cpp,jdye64/nifi-minifi-cpp,kevdoran/nifi-minifi-cpp