Enhance CONTRIBUTING with some basic information about setup of IDE

[laeubi]

I have read https://github.com/bndtools/bnd/blob/master/CONTRIBUTING.md

it says:

The only thing you need to build Bnd/Bndtools is Java.

alright that works, but I assume no one uses test-editor to develop the code and then maven to run the build.

So I downloaded latest, eclipse (for java developers) and installed latest bndtools release, then imported projects from the git repository, but it seems not enough, all projects show a lot of build errors.

So I'd like to suggest to enhance the CONTRIBUTING file with a small section about IDE setup.

[bjhargrave]

A PR would be welcome.

[chrisrueger]

Some info can be found here: https://bndtools.org/development.html