Mojo: Update/add README.md files.

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 in
+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).
+
+This library depends on the Environment library.
+
+Environment
+-----------
+
+The environment/ subdirectory contains a support (static) library that
+represents shared state needed to support the Bindings and GLES2 libraries.
+
+This library depends on the Utility library.
+
+
+GLES2
+-----
+
+The gles2/ subdirectory contains C++ wrappers (and some additional helpers) of
+the API defined in mojo/public/c/gles2/ (which provides access to GLES2).
+
+These wrappers depend on the Environment library.
+
+Shell
+-----
+
+The shell/ subdirectory contains a support (static) library that aids in writing
+Mojo applications and interacting with the Shell service.
+
+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.
+
+Utility
+-------
+
+The utility/ subdirectory contains a support (static) library that provides
+various basic functionality. Most notably, it provides an implementation of a
+RunLoop based on MojoWaitMany() that applications may use as the basis for
+asynchronous message processing.