Add contribution guideline (#1320)

.gitattributes

@@ -2,6 +2,7 @@
 
 /.editorconfig      export-ignore
 /.github            export-ignore
+/CONTRIBUTING.md    export-ignore
 /tests              export-ignore
 /.gitattributes     export-ignore
 /.gitignore         export-ignore

CONTRIBUTING.md

@@ -0,0 +1,80 @@
+# Contributing to Intervention Image
+
+Thank you for your interest in contributing to the project. Any contribution
+that improves the project is welcome and will be considered for integration.
+
+Please understand that there is no guarantee that your pull request will be
+accepted. To avoid misunderstandings, it is recommended that you first initiate
+a discussion about your planned changes on the [issue
+board](https://github.com/Intervention/image/issues).
+
+You agree that your contributions will be licensed under the same license as
+the main project. Before submitting a pull request familiarize yourself with
+the [license](LICENSE) and understand its terms and conditions.
+
+## Requirements
+
+To ensure a smooth collaboration process and maintain the quality of the
+project the submitted pull request must meet the following requirements to be
+accepted for review. It is a good idea to check these locally **before** you
+submit the PR.
+
+### Follow the Branching Model
+
+Please submit only one feature or bug fix per pull request. The best practice
+is to split your contributions into thematically meaningful branches and then
+submit them individually as pull requests.
+
+### Comply with the Coding Standard
+
+The project follows the [PSR12 coding
+standard](https://www.php-fig.org/psr/psr-12/) with some [additional
+extensions](phpcs.xml.dist). Adherence to these rules is essential and can be
+easily checked with the following command.
+
+```bash
+./vendor/bin/phpcs
+```
+
+Or by using the project's Docker environment.
+
+```bash
+docker-compose run --rm --build standards
+```
+
+### Write Tests
+
+For new features or changes to the existing code base, it is essential to cover
+them with tests. Integrate your tests in the  the project's test environment
+and consider both positive and negative scenarios.
+
+Make sure that all tests are passed with the following command.
+
+```bash
+./vendor/bin/phpunit
+```
+
+Or by using the project's Docker environment.
+
+```bash
+docker-compose run --rm --build tests
+```
+
+### Pay attention to the Analyzer Results
+
+A static analyzer is also used to avoid bugs and ensure quality. In addition to
+testing, it must also run without errors.
+
+Check the analyzer by running the following command.
+
+```bash
+./vendor/bin/phpstan analyze --memory-limit=512M --level=4 ./src
+```
+
+Or by using the project's Docker environment.
+
+```bash
+docker-compose run --rm --build analysis
+```
+
+**Thank you!**