Update README.markdown

Elaborate on subtree, change example code to use CMAKE_CURRENT_SOURCE_DIR
Adding/fixing author notice for the files [heavily] edited/created by me
Bugfix: C/CXX flags shouldn't be set for targets whose linker language are neither
better wording for the function descriptions
improving clang detection mechanism by distinguishing between C and C++
adding a function which returns the pedantic flags for the compiler enabled for
a given language and rewriting check_compile_flags so as to make it easier to use
Complete revamp of the tools for disabling compiler extensions
Cmake has native variables for toolchain detection with very asymmetrical names.
This little wrapper should make it less awkward to write toolchain dependent behavior.

--HG--
rename : DetectClang.cmake => DetectToolchain.cmake
Adding distinction between toolchains and function to return extra compiler
warnings flags for a given language
Complete revamp of the tools for enabling extra compiler warnings
Merging upstream