Added CONTRIBUTING.md

Signed-off-by: Patrick Cozzi <pjcozzi@gmail.com>

CONTRIBUTING.md

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+This document describes how we merge pull requests.  See the [Contributor's Guide](https://github.com/AnalyticalGraphicsInc/cesium/wiki/Contributor%27s-Guide) for getting the code and setting of a development environment. 
+
+We love pull requests.  We promptly [review](https://github.com/AnalyticalGraphicsInc/cesium/wiki/Code-Review-Tips) them, provide feedback, and merge.  Before we can merge, we require either:
+
+1. A [Contributor License Agreement](http://producingoss.com/en/copyright-assignment.html#copyright-assignment-cla) (CLA).  This can be emailed to cla@agi.com, and only needs to be completed once.  The CLA ensures that you retain copyright to your contributions, and that we have the right to use them and incorporate them into Cesium.  There is a CLA for [individuals](http://www.agi.com/licenses/individual-cla-agi-v1.0.txt) and [corporations](http://www.agi.com/licenses/corporate-cla-agi-v1.0.txt).  Please email completed CLAs and related questions to cla@agi.com.
+2. Every commit is signed-off on to indicate agreement to the Developer Certificate of Origin originally (DCO) developed for the Linux kernel.  The DCO text is here: [SubmittingPatches](https://github.com/wking/signed-off-by/blob/ab5bce80ad2259b47202b28905efff0d04032709/Documentation/SubmittingPatches).  Like the CLA, you retain copyright to your contributions, and that we have the right to use them and incorporate them into Cesium.  A sign-off looks like:
+```
+Signed-off-by: First-name Last-name <email-address>
+```
+To sign-off on commits, first make sure your name and email are setup with git:
+```
+git config --global --add user.name "First-name Last-name"
+git config --global --add user.email "email-address"
+```
+Then include the `-s` option with every commit, e.g.,
+```
+git commit -s -m 'commit message as before`
+```
+
+To keep our code quality high, please make sure:
+* Your code follows the [coding conventions](https://github.com/AnalyticalGraphicsInc/cesium/wiki/JavaScript-Coding-Conventions).
+* Your code passes [JSHint](http://www.jshint.com/).  We use the JSHint Eclipse plugin so it runs automatically when we save.  You can also run the `jsHint` Ant task from the command line.
+* To include tests with excellent code coverage for new features.  We use [Jasmine](http://pivotal.github.com/jasmine/) for writing tests.  Run them by browsing to [http://localhost:8080/Specs/SpecRunner.html](http://localhost:8080/Specs/SpecRunner.html).  Verify all new and existing tests pass.  For bonus points, test Chrome, Firefox, and other browsers supporting WebGL.
+* To update [LICENSE.md](LICENSE.md) if third-party libraries were added/updated/removed, including new version of existing libraries.  Mention it in [CHANGES.md](CHANGES.md).
+
+If new public classes, functions, or properties were added, also:
+   * Include reference documentation with code examples.  Check out the [best practices](https://github.com/AnalyticalGraphicsInc/cesium/wiki/Documentation-Best-Practices).
+   * Update [CHANGES.md](CHANGES.md).
+   * If the change is significant, add a new [Sandcastle](http://cesium.agi.com/Cesium/Apps/Sandcastle/index.html) example or extend and existing one.