Added CONTRIBUTING.md and updated contribution guidelines.

Author: beardyman

README.md

@@ -118,20 +118,3 @@ try {
 * If you specify a stored template and also provide inline content via `html` or `text`, you will receive an error.
 * By default, open and click tracking are enabled for a transmission.
 * By default, a transmission will use the published version of a stored template.
-
-## Development
-
-### Setup
-Run `composer install` inside the directory to install dependecies and development tools.
-
-### Testing
-Once all the dependencies are installed, you can execute the unit tests using:
-```
-composer test
-```
-
-### Contributing
-1. Check for open issues or open a fresh issue to start a discussion around a feature idea or a bug.
-2. Fork [the repository](http://github.com/SparkPost/php-sparkpost) on GitHub to start making your changes to the **master** branch (or branch off of it).
-3. Write a test which shows that the bug was fixed or that the feature works as expected.
-4. Send a pull request and bug the maintainer until it gets merged and published. :) Make sure to add yourself to [AUTHORS](https://github.com/SparkPost/php-sparkpost/blob/master/AUTHORS.md).