Create a CONTRIBUTING.md

[yrodiere]

There's no file specifically about helping new contributors, which is a shame. Also, it's inconsistent with other projects (ORM, Validator, Search).

Let's create one!

There are two approaches:

• Hibernate Search provides all the info, from license, to what to contribute, to relevant links, to how to build the project: https://github.com/hibernate/hibernate-search/blob/main/CONTRIBUTING.md
• Hibernate ORM does that, minus how to build the project, which sits in README.md instead: https://github.com/hibernate/hibernate-orm/blob/main/CONTRIBUTING.md

Either approach is fine, though my personal preference goes to the first.

[DavideD]

The README.md should contain already the information to build the project and run tests.

What I think it's missing is a guideline about how the code is structured and some tips and configuration properties that could be useful for debugging and testing.