* CMake: Prefix options with "YAML" and hide platform-specific options
When including yaml-cpp as a subproject, some option names can conflict with other projects.
(1) Make sure the yaml-cpp options are prefixed with YAML
(2) Hide platform-specific options when possible to avoid cluttering the cmake option list
* Update docs for change from BUILD_SHARED_LIBS to YAML_BUILD_SHARED_LIBS
In the second step, I treat `source directory` as the `src` directory.
Though my version seems a little verbose, it's at least unambiguous. I think it's important to make it clear whether the user should be.
Feel free to make modifications.
