doc: Use README.md as mainpage

Author: gjasny

doc/Doxyfile

@@ -5,8 +5,9 @@ EXCLUDE_SYMBOLS        = prometheus::detail::*
 FILE_PATTERNS          = *.h *.cc
 GENERATE_LATEX         = NO
 GRAPHICAL_HIERARCHY    = NO
-INPUT                  = ../core/include ../core/src ../pull/include ../pull/src ../push/include ../push/src
+INPUT                  = ../README.md ../core/include ../core/src ../pull/include ../pull/src ../push/include ../push/src
 RECURSIVE              = YES
 SHOW_FILES             = NO
 SHOW_INCLUDE_FILES     = NO
 SHOW_USED_FILES        = NO
+USE_MDFILE_AS_MAINPAGE = ../README.md