First of all, thank you for contributing, you are awesome!
Here are a few rules to follow in order to ease code reviews, and discussions before maintainers accept and merge your work.
As Symfony follows the next standards: PSR-0, PSR-1, PSR-2, PSR-4. You also MUST follow them.
You MUST follow the Symfony Coding Standards.
If you don't know about any of them, you should really read the recommendations.
You can use a helpful tool phpcs with a last version of Symfony standards.
Or you can use a PHP-CS-Fixer tool to fix all coding style issues.
You MUST run the test suite.
You MUST write (or update) unit tests.
You SHOULD write documentation.
Please, write commit messages that make sense, and rebase your branch before submitting your Pull Request.
One may ask you to squash your commits too.
This is used to "clean" your Pull Request before merging it (we don't want commits such as fix tests, fix 2, fix 3, etc.).
Thank you!