fixed main page (from src/HEADER.md)

doc/Doxyfile

@@ -1109,7 +1109,7 @@ FILTER_SOURCE_PATTERNS =
 # (index.html). This can be useful if you have a project on for instance GitHub
 # and want to reuse the introduction page also for the doxygen output.
 
-USE_MDFILE_AS_MAINPAGE = Readme.md
+USE_MDFILE_AS_MAINPAGE = src/HEADER.md
 
 #---------------------------------------------------------------------------
 # Configuration options related to source browsing