Add CONTRIBUTING

Author: picodexter

CONTRIBUTING.md

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+# Contributing
+
+You are very welcome to contribute to this project by giving constructive criticism, feedback, creating issues and
+submitting pull requests.
+
+Each contribution is appreciated.
+
+## Areas of Focus
+
+Every aspect of this project from documentation to actual code is subject for improvement.
+
+However, due to the security aspect we would like to strongly encourage reviews of the implementations of the
+encryption and decryption processes. This also includes the choice of supported ciphers and modes.
+
+## Bug Reports
+
+Bugs may be reported in the project issue tracker.
+
+Bug reports should include information on
+
+*   the version of the Symfony framework
+*   the version of the ParameterEncryptionBundle
+*   the version of a potentially affected add-on bundle (like ParameterEncryptionDefuseBundle)
+*   instructions on how to reproduce the problem
+*   the expected behavior
+*   the actual behavior
+
+We may also ask for sample code in a fresh project based on the
+[Symfony Standard Edition](https://github.com/symfony/symfony-standard) if we cannot simulate it otherwise.
+Ideally, this would be as easy as cloning a repository and following a couple of steps.
+
+## Contributing Code
+
+Please make sure that the documentation is still up to date and not missing any vital information relating to your
+changes!
+
+### Before Submitting a Pull Request
+
+In order to maintain a certain quality level for the code, this project is using several tools to help with testing and
+coding standards.
+
+The following steps are required:
+
+*   All tests have to execute successfully.
+
+*   Check the code coverage in order to stay at 100%:
+
+    ```console
+    $ ./vendor/bin/phpunit -v --coverage-text
+    ```
+
+*   Run PHP CS Fixer for coding standards / code style:
+
+    ```console
+    $ ./vendor/bin/php-cs-fixer fix -v --dry-run --diff .
+    ```
+
+Automated versions of these checks will run during the build process.
+
+Any errors will make the build fail.
+
+You can find links to the results of the build process and the code coverage at the top of the [README](README.md).