[boost] CMake documentation (mostly) available

I've written a document that explains how to use CMake to build or consume Boost, at https://github.com/boostorg/cmake/blob/develop/README.md This targets users of Boost. Developer-centric documentation is also available at https://github.com/boostorg/cmake/blob/develop/developer.md This is still unfinished, but I decided to "publish" it because the portions that are present are still (hopefully) valuable.