Added CONTRIBUTING.md and updated README.md

Author: vladar

CONTRIBUTING.md

@@ -0,0 +1,27 @@
+# Contributing to GraphQL PHP
+
+## Workflow
+
+If your contribution requires significant or breaking changes, or if you plan to propose a major new feature,
+we recommend you to create an issue on the [Github](https://github.com/webonyx/graphql-php/issues) with 
+a brief proposal and discuss it with us first.
+
+For smaller contributions just use this workflow:
+
+* Fork the project.
+* Add your features and or bug fixes.
+* Add tests. Tests are important for us.
+* Send a pull request
+
+## Using GraphQL PHP from a Git checkout
+```
+$ git clone https://github.com/webonyx/graphql-php.git
+$ cd graphql-php
+$ composer install
+```
+
+## Running tests
+From the project root:
+```
+$ ./bin/phpunit
+```