Update README and rename files to make it clearer that a build step is required.

Gruntfile.js

@@ -13,7 +13,7 @@ module.exports = function(grunt) {
     pkg: grunt.file.readJSON("package.json"),
 
     eslint: {
-      src: ["browser-polyfill.js", "Gruntfile.js"],
+      src: ["browser-polyfill.in.js", "Gruntfile.js"],
     },
 
     replace: {
@@ -41,7 +41,7 @@ module.exports = function(grunt) {
           {
             expand: true,
             flatten: true,
-            src: ["browser-polyfill.js"],
+            src: ["browser-polyfill.in.js"],
             dest: "dist/",
           },
         ],

README.md

@@ -7,6 +7,21 @@ Chrome.
 
 [w3-browserext]: https://www.w3.org/community/browserext/
 
+## Building
+
+To build, assuming you're already installed [npm](https://www.npmjs.com/),
+simply run:
+
+```sh
+npm install
+grunt
+```
+
+This will build both non-minified and minified versions of the final library,
+and output them to `dist/browser-polyfill.js` and `dist/browser-polyfill.min.js`,
+respectively.
+
+
 ## Basic Setup
 
 In order to use the polyfill, it must be loaded into any context where