Mojo: Update/add README.md files.

mojo/public/c/README.md

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+Mojo Public C API
+=================
+
+This directory contains C language bindings for the Mojo Public API.
+
+Environment
+-----------
+
+The environment/ subdirectory defines some common things that, while not part of
+the system API, may be required for GLES2 (for example). These are things that a
+Mojo application may be required to provide to the GLES2 (for example) library
+in order to use it. (However, the Mojo application may implement these things as
+it sees fit.)
+
+GLES2
+-----
+
+The gles2/ subdirectory defines the GLES2 C API that's available to Mojo
+applications. To use GLES2, Mojo applications must link against a dynamic
+library (the exact mechanism being platform-dependent) and use the header files
+in this directory as well as the standard Khronos GLES2 header files.
+
+The reason for this, rather than providing GLES2 using the standard Mojo IPC
+mechanism, is performance: The protocol (and transport mechanisms) used to
+communicate with the Mojo GLES2 service is not stable nor "public" (mainly for
+performance reasons), and using the dynamic library shields the application from
+changes to the underlying system.
+
+System
+------
+
+The system/ subdirectory provides definitions of the basic low-level API used by
+all Mojo applications (whether directly or indirectly). These consist primarily
+of the IPC primitives used to communicate with Mojo services.
+
+Though the message protocol is stable, the implementation of the transport is
+not, and access to the IPC mechanisms must be via the primitives defined in this
+directory.
+
+Test Support
+------------
+
+This directory contains a C API for running tests. This API is only available
+under special, specific test conditions. It is not meant for general use by Mojo
+applications.