Update README.md with implementation docs (#78)

PR-URL: #78
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

Author: KevinEady

README.md

@@ -1,7 +1,17 @@
 Node.js Addon Examples
 =========================================
+**A repository of [Node.js Addons](https://nodejs.org/api/addons.html#addons_c_addons) examples.**
 
-This repository is the home of the Addon Examples.
+Implementations of examples are named either after Node.js versions (`node_0.10`,
+`node_0.12`, etc), or Node.js addon implementation APIs:
+- [`nan`](https://github.com/nodejs/nan): C++-based abstraction between Node and direct V8 APIs.
+- [`napi`](https://nodejs.org/api/n-api.html): C-based API guaranteeing [ABI stability](https://nodejs.org/en/docs/guides/abi-stability/) across different node versions as well as JavaScript engines.
+- [`node-addon-api`](https://github.com/nodejs/node-addon-api): header-only C++ wrapper classes which simplify the use of the C-based N-API.
+
+Implementations against unsupported versions of Node.js are provided for
+completess and historical context. They are not maintained.
+
+## Usage
 
 The directory structure is as follows:
 
@@ -17,12 +27,6 @@ The directory structure is as follows:
   .
 ```
 
-Implementations are named either after Node.js versions (`node_0.10`,
-`node_0.12`, etc), or Node.js addon implementation APIs (`nan`, `napi`,
-or `node-addon-api`).
-
-Implementations against unsupported versions of Node.js are provided for
-completess and historical context. They are not maintained.
 
 In each example's implementation subdirectory, run