docs: add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to OpenJUMP
+
+OpenJUMP (OJ) is maintained by a very small team of volunteers, so any contribution is welcome.
+
+## Setting up a development environment
+
+OJ uses Maven and works with Eclipse, IntelliJ IDEA, or any Maven-aware Java IDE. For a full
+walkthrough of setting up **OJ Core** or the **HelloWorld Extension**, see the wiki:
+
+https://ojwiki.soldin.de/index.php?title=Eclipse:_Set_up_project_and_example_extension_from_git_sources
+
+To build from the command line:
+
+```
+mvn -B package -P snapshot
+```
+
+This is also what CI runs on every push, so it's the quickest way to confirm your change builds
+cleanly before opening a PR.
+
+## Before opening a pull request
+
+- **Build and run your change locally first.** A PR without at least a local `mvn -B package -P
+  snapshot` pass (and, for anything touching the UI, an actual run of the built application)
+  isn't ready for review yet.
+- **Keep it scoped.** One logical change per PR — please don't bundle an unrelated fix, refactor,
+  or formatting pass in with the change you're actually proposing. Smaller PRs are easier to
+  review and easier to revert if something goes wrong.
+- **Reference the issue it addresses**, where one exists, in the PR description.
+- If you're fixing a bug reported a while ago, it's worth confirming it still reproduces against
+  current `main` first — some issues predate later refactors and may already be partially or
+  fully resolved.
+
+## Issues
+
+Bug reports and proposals are welcome as [GitHub issues](https://github.com/openjump-gis/openjump/issues). 
+Please include the OpenJUMP version, your operating system and Java version, and the steps to 
+reproduce the problem. 
+If you've already found and fixed something, feel free to open a [pull request](https://github.com/openjump-gis/openjump/pulls) 
+referencing the issue.
+
+## License
+
+OpenJUMP is licensed under the GNU General Public License v2 (GPLv2). When submitting a
+contribution, make sure you agree to it being distributed under the same license.
+
+## Extensions
+
+If your contribution is really a standalone plugin rather than a change to OJ Core, consider
+setting up a separate extension repository instead — see the 
+[README's Extensions section](https://github.com/openjump-gis/openjump/blob/main/README.md#extensions)
+for the naming convention (`xxx-extension`, `xxx-driver`) and the
+[HelloWorldExtension repo](https://github.com/openjump-gis/helloworld-extension) for an example.