Add main README for jdt.debug and remove obsolete CONTRIBUTING file

The content is based on the content of the corresponding files in
jdt.core/ui.

README.md

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+# JDT Debug
+
+JDT Debug implements Java debugging support and works with any JDPA-compliant target Java VM.
+It is implemented on top of the language independent "debug model" provided by the platform debugger. 
+JDT debug provides the following debugging features:
+
+- Launching of a Java VM in either run or debug mode
+- Attaching to a running Java VM
+- Expression evaluation in the context of a stack frame
+- Scrapbook pages for interactive Java code snippet evaluation
+- Dynamic class reloading where supported by Java virtual machine
+
+For more information about JDT Debug refer to the [JDT Debug website](https://eclipse.dev/eclipse/debug/index.php).
+For general information about JDT and important links, refer to the [JDT wiki page](https://github.com/eclipse-jdt/eclipse.jdt.core/wiki) or the [JDT project overview page](https://projects.eclipse.org/projects/eclipse.jdt).
+
+## Contributing
+
+[Contributions are always welcome!](https://github.com/eclipse-jdt/.github/blob/main/CONTRIBUTING.md)
+
+Please bear in mind that this project is almost entirely developed by volunteers.
+If you do not provide the implementation yourself (or pay someone to do it for you), the bug might never get fixed.
+If it is a serious bug, other people than you might care enough to provide a fix.
+
+## License
+
+[Eclipse Public License (EPL) v2.0](https://www.eclipse.org/legal/epl-2.0/)
+
+## Links
+
+- https://github.com/eclipse-jdt/eclipse.jdt.core/wiki
+- https://github.com/eclipse-jdt/.github/blob/main/CONTRIBUTING.md
+- https://projects.eclipse.org/projects/eclipse.jdt