(docs): add a README describing physijs changes

- this makes it more explicit why it's been vendored in -- it has
  incompatible changes -- and makes more explicit what those changes
  are with documentation (instead of just commit messages)
  - it also explains why those changes were made, with lots of links
    as there's a decent bit of context around that
    - and this context is key to what made this library possible, so
      it's good to have finally documented it!

- also add the same worker links to the app root README for anyone
  who might want more info on those

- (pkg): modify package.json files config so that this new README
  isn't included in the package

README.md

@@ -30,7 +30,7 @@ npm install -S physijs-webpack
 var Physijs = require('physijs-webpack');
 ```
 
-and install `worker-loader` with:
+and install [`worker-loader`](https://github.com/webpack-contrib/worker-loader) with:
 
 ```bash
 npm install -D worker-loader
@@ -42,7 +42,7 @@ npm install -D worker-loader
 var Physijs = require('physijs-webpack/browserify');
 ```
 
-and install `webworkify` with:
+and install [`webworkify`](https://github.com/browserify/webworkify) with:
 
 ```bash
 npm install -D webworkify

package.json

@@ -4,7 +4,7 @@
   "description": "PhysiJS port for bundlers with out-of-the-box support for Webpack and Browserify",
   "main": "webpack.js",
   "files": [
-    "physijs/*",
+    "physijs/**/*.js",
     "webpack.js",
     "browserify.js",
     "browserify-worker-stub.js"

physijs/README.md

@@ -0,0 +1,18 @@
+# PhysiJS Changes
+
+The files here are 99.9% the same as the original [`physijs`](https://github.com/chandlerprall/Physijs) code.
+There are a few small modifications to make it compatible with bundlers and work out-of-the-box with zero config:
+
+- accepting the `PhysijsWorker` constructor function as an argument instead of just using `new Worker(...)`
+  - this allows for custom workers, such as those of [`worker-loader`](https://github.com/webpack-contrib/worker-loader) and [`webworkify`](https://github.com/browserify/webworkify)
+  - it also means one no longer has to set `Physijs.scripts.worker`
+- `require`ing `ammo.js` directly in the `physijs_worker.js` file instead of using [`importScripts`](https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Using_web_workers#Importing_scripts_and_libraries)
+  - this can only be done because of the extra functionality we get from a bundler and its custom worker
+  - this means one no longer has to set `Physijs.scripts.ammo`
+- slightly change the `Ammo` code so it doesn't try to `require` anything in its Emscripten header
+  - Emscripten (or at least the old version that was used to create Physijs's old vendored `ammo.js`) output both web and node code in one file, so it caused issues when being parsed by bundlers (can read more [here](https://github.com/agilgur5/physijs-webpack/commit/0174f2c073f62684c764b83cacf66102445d2218))
+
+[`silviopaganini/physijs-browserify`](https://github.com/silviopaganini/physijs-browserify), which this library was originally based off, also made some changes:
+
+- exporting as a CJS module with `module.exports` instead of a global with `window.Physijs`
+- accepting [`THREE`](https://github.com/mrdoob/three.js) as an argument instead of assuming it exists as a global