[wip/metapod] Document optimization flags for robotpkg users.

metapod/DESCR

@@ -9,4 +9,5 @@ As of today, the following algorithms have been implemented:
 This software is designed to be a low-level library, performance is
 thus a key issue. For increased performance, it is strongly
 recommended to add the "-march=native -mfpmath=sse" flags to the
-CMAKE_CXX_FLAGS variable before compiling it.
\ No newline at end of file
+CXXFLAGS.metapod variable in $ROBOTPKG_BASE/etc/robotpkg.conf before
+installing it.