Doxygen: fixup main page

To avoid the {#mainpage} tag to be rendered on GitHub, explicitly add a
`\mainpage` section to `rapidjson.h`, referring to `readme.md`.

include/rapidjson/rapidjson.h

@@ -10,6 +10,10 @@
 	\todo Complete Doxygen documentation for configure macros.
  */
 
+/*! \mainpage
+	Documentation can be found in \ref readme.md README.
+*/
+
 #include <cstdlib>	// malloc(), realloc(), free()
 #include <cstring>	// memcpy()
 

readme.md

-# RapidJSON {#mainpage}
+# RapidJSON
 
 Copyright (c) 2011-2014 Milo Yip (miloyip@gmail.com)