add contributing.md

[closes #513]

contributing.md

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+# Contributing
+
+If you want to fix a bug or implement a feature, you are welcome to do so. But read this first before implementing something.
+
+Possible contributions:
+
+- bugfixes (visual bugs, documentation bugs, API bugs),
+- modifications to existing components,
+- adding more documentation,
+- implementing new components.
+
+Please always open an issue first to be sure your work will be accepted (with the exception to documentation bugs). Especially all feature development (changes/new components/API) has to be discussed first. Thank you for understanding.
+
+## Testing code changes locally
+
+- PHPStan checks have to pass, run `composer phpstan`.
+- Unit & integration have to pass, run `composer tests`.
+- Commit generated/changed SQLs for all tests. These SQL files are generated only for Postgres, so it is required to run tests at least for this database.
+
+To set up tests configuration, copy [databases.sample.ini](tests/databases.sample.ini) file, name it `database.ini` and updated sections with proper database connection credentials. You may comment out those sections not to run them locally.