Document CFLAGS, CXXFLAGS, ASFLAGS and CPPFLAGS.

After understanding the difference between CXXFLAGS and CPPFLAGS, this
is a revised commit for https://github.com/sudar/Arduino-Makefile/pull/257
Merge pull request #263 from peplin/document-flags-vars

Document CFLAGS, CXXFLAGS, ASFLAGS and CPPFLAGS.