chore: add README.md to each plunker

closes #1974

Author: Foxandxss

public/docs/_examples/plunker.README.md

@@ -0,0 +1,2 @@
+### Angular 2 Documentation Example 
+

tools/plunker-builder/plunkerBuilder.js

@@ -10,10 +10,10 @@ var mkdirp = require('mkdirp');
 var indexHtmlTranslator = require('./indexHtmlTranslator');
 var regionExtractor = require('../doc-shredder/regionExtractor');
 var COPYRIGHT, COPYRIGHT_JS_CSS, COPYRIGHT_HTML;
+var README; // content of plunker.README.md for plunkers
 var SYSTEMJS_CONFIG; // content of systemjs.config.js for plunkers that use systemjs
 var TSCONFIG;  // content of tsconfig.json for plunkers that use systemjs
 
-
 module.exports = {
   buildPlunkers: buildPlunkers
 };
@@ -30,7 +30,7 @@ function buildCopyrightStrings() {
 }
 
 function buildPlunkers(basePath, destPath, options) {
-  getSystemJsConfigPlunker(basePath);
+  getPlunkerFiles(basePath);
   var errFn = options.errFn || function(e) { console.log(e); };
   var plunkerPaths = path.join(basePath, '**/*plnkr.json');
   var fileNames = globby.sync(plunkerPaths, { ignore: "**/node_modules/**"});
@@ -59,7 +59,7 @@ function buildPlunkerFrom(configFileName, basePath, destPath) {
   try {
     var config = initConfigAndCollectFileNames(configFileName);
     var postData = createPostData(config);
-    addSystemJsConfig(config, postData);
+    addPlunkerFiles(config, postData);
     var html = createPlunkerHtml(postData);
     fs.writeFileSync(outputFileName, html, 'utf-8');
     if (altFileName) {
@@ -81,19 +81,29 @@ function buildPlunkerFrom(configFileName, basePath, destPath) {
   }
 }
 
-/**
- * Add plunker versions of systemjs.config and tsconfig.json
- */
-function addSystemJsConfig(config, postData){
+function addPlunkerFiles(config, postData) {
+  addReadme(config, postData);
   if (config.basePath.indexOf('/ts') > -1) {
      // uses systemjs.config.js so add plunker version
      postData['files[systemjs.config.js]'] = SYSTEMJS_CONFIG;
      postData['files[tsconfig.json]'] = TSCONFIG;
   }
 }
 
-function getSystemJsConfigPlunker(basePath) {
+function addReadme(config, postData) {
+  var existingFiles = config.fileNames.map(function(file) {
+    return file.substr(file.lastIndexOf('/') + 1);
+  });
+
+  if (existingFiles.indexOf('README.md') === -1) {
+    var plunkerReadme = README + config.description;
+    postData['files[README.md]'] = plunkerReadme;
+  }
+}
+
+function getPlunkerFiles(basePath) {
   // Assume plunker version is sibling of node_modules version
+  README = fs.readFileSync(basePath +  '/plunker.README.md', 'utf-8');
   SYSTEMJS_CONFIG = fs.readFileSync(basePath + '/systemjs.config.plunker.js', 'utf-8');
   SYSTEMJS_CONFIG +=  COPYRIGHT_JS_CSS;
   TSCONFIG = fs.readFileSync(basePath + '/tsconfig.json', 'utf-8');