doc: Add initial documentation for N-API

Add the initial documentation for the N-API

This PR is a result of work in the abi-stable-node repo:
https://github.com/nodejs/abi-stable-node/tree/doc,
with this PR being the cumulative work on the documentation
sections in that repo with the following contributors
in alphabetical order:

Author: Arunesh Chandra <arunesh.chandra@microsoft.com>
Author: Gabriel Schulhof <gabriel.schulhof@intel.com>
Author: Hitesh Kanwathirtha <hiteshk@microsoft.com>
Author: Jason Ginchereau <jasongin@microsoft.com>
Author: Michael Dawson <michael_dawson@ca.ibm.com>
Author: Sampson Gao <sampsong@ca.ibm.com>
Author: Taylor Woll <taylor.woll@microsoft.com>

PR-URL: #12549
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/_toc.md

@@ -8,6 +8,7 @@
 * [Assertion Testing](assert.html)
 * [Buffer](buffer.html)
 * [C/C++ Addons](addons.html)
+* [C/C++ Addons - N-API](n-api.html)
 * [Child Processes](child_process.html)
 * [Cluster](cluster.html)
 * [Command Line Options](cli.html)

doc/api/addons.md

@@ -213,6 +213,26 @@ Addon developers are recommended to use to keep compatibility between past and
 future releases of V8 and Node.js. See the `nan` [examples][] for an
 illustration of how it can be used.
 
+
+## N-API
+
+> Stability: 1 - Experimental
+
+N-API is an API for building native Addons. It is independent from
+the underlying JavaScript runtime (ex V8) and is maintained as part of
+Node.js itself. This API will be Application Binary Interface (ABI) stable
+across version of Node.js. It is intended to insulate Addons from
+changes in the underlying JavaScript engine and allow modules
+compiled for one version to run on later versions of Node.js without
+recompilation. Addons are built/packaged with the same approach/tools
+outlined in this document (node-gyp, etc.). The only difference is the
+set of APIs that are used by the native code. Instead of using the V8
+or [Native Abstractions for Node.js][] APIs, the functions available
+in the N-API are used.
+
+The functions available and how to use them are documented in the
+section titled [C/C++ Addons - N-API](n-api.html).
+
 ## Addon examples
 
 Following are some example Addons intended to help developers get started. The