docs(aio): complete the plunker-builder docs (#18151)

PR Close #18151

aio/tools/plunker-builder/README.md

@@ -1,10 +1,54 @@
 # Overview
 
-In the AIO application, we can embed a running version of the example as a [Plunker](http://plnkr.co/). We can also provide a
-link to create a runnable version of the example in the [Plunker](http://plnkr.co/edit) editor.
+[Plunker](http://plnkr.co) is an online tool for creating, collaborating and sharing ideas. In AIO we use it to share one or more runnable versions of our examples.
 
-This folder contains three utilities:
+Plunker comes in two flavours. The [classic UI](http://plnkr.co/edit) and a [embedded UI](http://embed.plnkr.co). The latter can be used both in a new tab or embedded within a guide. The AIO project uses the embedded version in both ways.
 
-* regularPlunker.js - generates an HTML file for each example that will post to Plunker to create a new editable project, when rendered.
-* embeddedPlunker.js - generates an HTML file for each example that can be used in an iframe to render an embedded Plunker project.
-* generatePlunkers.js - executes each of the `regularPlunker.js` and `embeddedPlunker.js` utilities to generate all the example plunker files.
+## Plunker generation
+
+Both flavours are created within `builder.js`. How is a plunker created? What is the process from a directory with files to a link with a plunker.
+
+An "executable" plunker is an HTML file with a `<form>` that makes a post to plunker on submit. It contains an `<input>` element for each file we need in the plunker.
+
+So the `builder.js` job is to get all the needed files from an example and build this HTML file for you.
+
+For plunkers, we use an special `systemjs.config` that exists in `/aio/tools/examples/shared/boilerplate/src/systemjs.config.web.js` and we also add the Google's copyright to each file.
+
+## Customizing the generation per example basis
+
+How do this tool know about what is an example and what is not? It will look for all folders containing a `plnkr.json` file. If found, all files within the folder and subfolders will be used in the plunker, with a few generic exceptions that you can find at `builder.js`.
+
+You can use the `plnkr.json` to customize the plunker generation. For example:
+
+```json
+{
+  "description": "Tour of Heroes: Part 6",
+  "basePath": "src/",
+  "files":[
+    "!**/*.d.ts",
+    "!**/*.js",
+    "!**/*.[1,2].*"
+  ],
+  "tags": ["tutorial", "tour", "heroes", "http"]
+}
+```
+
+Here you can specify a description for the plunker, some tags, a basePath and also a files array where you can specify extra files to add or to ignore.
+
+## Classic plunkers and embedded ones
+
+Luckily, both kind of plunkers are very similar, the are created in the same way with a minor exceptions.
+
+To handle those exceptions, we have the `embeddedPlunker.js` and the `regularPlunker.js`. Thanks to them, the `builder.js`  is as generic as possible.
+
+## Executing the plunker generation
+
+`generatePlunkers.js` will create a classic plunker and a embedded plunker for each `plnk.json` it finds.
+
+Where? at `src/generated/live-examples/`.
+
+Then the `<live-example>` embedded component will look at this folder to get the plunker it needs for the example
+
+## Appendix: Why not generating plunkers on runtime?
+
+At AngularJS, all the plunkers were generated a runtime. The downside is that all the example codes would need to be deployed as well and they won't be no longer useful after the plunker is generated. This tool takes a few seconds to run, and the end result is only 3mb~