Create CONTRIBUTING.md

Author: stefanak-michal

CONTRIBUTING.md

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+# Contributing
+We'd love for you to contribute to our source code and to make php Bolt library even better than it is today! Here are the guidelines we'd like you to follow.
+
+## Code of Conduct
+Help us keep this project open and inclusive. Please read and follow our [Code of Conduct](https://github.com/neo4j-php/Bolt/blob/master/CODE_OF_CONDUCT.md).
+
+## Did you find a bug?
+Ensure the bug was not already reported by searching on GitHub under [Issues](https://github.com/neo4j-php/Bolt/issues?q=is%3Aissue).
+
+If you're unable to find an open issue addressing the problem, [open a new one](https://github.com/neo4j-php/Bolt/issues/new/choose). Be sure to include a title and clear description, as much relevant information as possible, and a code sample or an executable test case demonstrating the expected behavior that is not occurring.
+
+## Did you write a piece of code?
+Open a new GitHub pull request with the patch. Don't forget to run phpunit when you make changes.
+
+Ensure the PR description clearly describes the problem and solution. Include the relevant issue number if applicable.
+
+## Clean code
+We like to keep the code unified and clean as possible.
+
+- Follow PHP Standards Recommendations (PSR)
+- Avoid import with `use function <php_function>;`
+- Use type specification for method arguments
+- Set methods return type
+- Add descriptive method annotations if needed
+- Don't use @deprecated annotation
+- Follow Bolt specification by [official documentation](https://7687.org/)
+
+\
+PHP Bolt library is a volunteer effort. We are thankful for your interest or contribution.