Add contributing file

CONTRIBUTING.md

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+# How to Contribute
+
+## Pull Requests
+
+1. Fork the Slim Framework repository
+2. Create a new branch for each feature or improvement
+3. Send a pull request from each feature branch to the **develop** branch
+
+It is very important to separate new features or improvements into separate feature branches, and to send a
+pull request for each branch. This allows me to review and pull in new features or improvements individually.
+
+## Style Guide
+
+All pull requests must adhere to the [PSR-2 standard](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md).
+
+## Unit Testing
+
+All pull requests must be accompanied by passing unit tests and complete code coverage. The Slim Framework uses phpunit for testing.
+
+[Learn about PHPUnit](https://github.com/sebastianbergmann/phpunit/)