Eliminate warning about missing readme.txt, use readme.md

dox

@@ -661,7 +661,7 @@ WARN_LOGFILE           =
 # directories like "/usr/src/myproject". Separate the files or directories
 # with spaces.
 
-INPUT                  = tinyxml2.h xmltest.h readme.txt
+INPUT                  = tinyxml2.h xmltest.h readme.md
 
 # This tag can be used to specify the character encoding of the source files
 # that doxygen parses. Internally doxygen uses the UTF-8 encoding, which is