Add initial API overview document.

OVERVIEW.md

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+# gRPC for Swift - Public API
+
+This document will provide an overview of the gRPC API for Swift.
+It follows a standard form used by each language-platform implementation.
+
+##Basic Functionality
+###_Choose a service definition proto to use for examples_
+###How is a New Stub Created?
+###Simple Request-Response RPC: Client-side RPC
+###Simple Request-Response RPC: Server Implementation of RPC
+###Show how Client does two RPCs sequentially 
+###Show how Client does two RPCs asynchronously
+###Any code for handling incoming RPC on server that might need to be written
+###Server Streaming RPC: Client-side code
+###Server Streaming RPC: Server-side code
+###How is a Server Created?
+
+##Advanced
+###RPC canceling on client side
+###Code to look for and handle cancelled RPC on Server side
+###Client Streaming RPC: Client-side code
+###Client Streaming RPC: Server-side code
+###Flow control interactions while sending & receiving messages
+###Flow control and buffer pool : Control API
+###Bi Directional Streaming : Client-side code
+###Bi Directional Streaming : Server-side code
+###Any stub deletion/cleanup code needed