Adds lots of Mojo documentation

mojo/public/cpp/README.md

-Mojo Public C++ API
-===================
-
-This directory contains C++ language bindings for the Mojo Public API.
-
-A number of subdirectories provide wrappers for the lower-level C APIs (in
-subdirectories of the same name, under mojo/public/c/). Typically, these
-wrappers provide increased convenience and/or type-safety.
-
-Other subdirectories provide support (static) libraries of various sorts. In
-this case, the organization is to have the public interface for the library
-defined in header files in the subdirectory itself and the implementation of the
-library at a lower level, under a lib (sub)subdirectory. A developer should be
-able to substitute their own implementation of any such support library, and
-expect other support libraries, which may depend on that library, to work
-properly.
-
-Bindings
---------
-
-The bindings/ subdirectory contains a support (static) library needed by the
-code generated by the bindings generator tool (in mojo/public/tools/bindings/),
-which translates Mojo IDL (.mojom) files into idiomatic C++ (among other
-languages).
-
-System
-------
-
-The system/ subdirectory contains C++ wrappers (and some additional helpers) of
-the API defined in mojo/public/c/system/, which defines the basic, "core" API,
-especially used to communicate with Mojo services.
-
-Test Support
-------------
-
-The test_support/ subdirectory contains C++ wrappers of the test-only API
-defined in mojo/public/c/test_support/. It is not meant for general use by Mojo
-applications.