Some basic CONTRIBUTING doc

[robertvazan]

A lot of useful information can be published in project's CONTRIBUTING.md.

Right now I am wrestling with formatter settings. My usual Java style is wildly different. I have my own custom formatter configured in Eclipse. I have configured WDTK projects to use "Eclipse [built-in]" formatter instead of my own and disabled import wildcards, but I still get large diffs when organizing imports (Ctrl+Shift+O) or formatting code (Ctrl+Shift+F), even for small selections of code. For example, import wildcards seem to be used over some threshold, but I don't know what is that threshold. Publishing standard IDE settings (or referencing existing ones on the Internet) would help reduce formatting noise without having to abandon convenient IDE features.