added building and contributing to readme

Author: liabru

README.md

@@ -103,6 +103,22 @@ Some of the demos are avaliable at [codepen.io/liabru](http://codepen.io/liabru/
 See the [Matter.js API Docs (v0.8.0)](http://brm.io/matter-js-docs/).
 <br>If you're using the [edge version (master)](https://raw2.github.com/liabru/matter-js/master/build/matter.js) then see the [API Docs (master)](http://brm.io/matter-js-docs-master/).
 
+### Building and Contributing
+
+To build you must first install [node.js](http://nodejs.org/) and [grunt](http://gruntjs.com/), then run
+
+	npm install
+
+This will install the required build dependencies, then run
+
+	grunt dev
+
+which is a task that builds the `matter-dev.js` file, spawns a `connect` and `watch` server, then opens `demo/dev.html` in your browser. Any changes you make to the source will automatically rebuild `matter-dev.js` and reload your browser for quick and easy testing.
+
+Contributions are welcome, please ensure they follow the same style and architecture as the rest of the code. You should run `grunt test` to ensure `jshint` gives no errors. Please do not include any changes to the files in the `build` directory. 
+
+If you'd like to contribute but not sure what to work on, feel free to message me. Thanks!
+
 ### Changelog
 
 To see what's new or changed in the latest version, see the [changelog](https://github.com/liabru/matter-js/blob/master/CHANGELOG.md)