Adds lots of Mojo documentation

mojo/public/README.md

-Mojo Public API
-===============
-
-The Mojo Public API is a binary stable API to the Mojo system.
-
-It consists of support for a number of programming languages (with a directory
-for each support language), some "build" tools and build-time requirements, and
-interface definitions for Mojo services (specified using an IDL).
-
-Note that there are various subdirectories named tests/. These contain tests of
-the code in the enclosing directory, and are not meant for use by Mojo
-applications.
-
-C/CPP/JS
---------
-
-The c/, cpp/, js/ subdirectories define the API for C, C++, and JavaScript,
-respectively.
-
-The basic principle for these directories is that they consist of the source
-files that one needs at build/deployment/run time (as appropriate for the
-language), organized in a natural way for the particular language.
-
-Interfaces
-----------
-
-The interfaces/ subdirectory contains Mojo IDL (a.k.a. .mojom) descriptions of
-standard Mojo services.
-
-Platform
---------
-
-The platform/ subdirectory contains any build-time requirements (e.g., static
-libraries) that may be needed to produce a Service library for certain
-platforms, such as a native shared library or as a NaCl binary.
-
-Tools
------
-
-The tools/ subdirectory contains tools that are useful/necessary at
-build/deployment time. These tools may be needed (as a practical necessity) to
-use the API in any given language, e.g., to generate bindings from Mojo IDL
-files.