[platform-ui-dev] New Platform UI contribution guide

[Stefan Xenos <sxenos@xxxxxxxxxx>]

I've updated the instructions for contributing to platform UI to describe the steps for installing via Oomph:

https://wiki.eclipse.org/Platform_UI/How_to_Contribute

I would appreciate feedback on the guide and the setup process itself.

There's still some things I'm aware of that I'd still like to fix:

- The oomph install is pulling in mylyn.

- It isn't setting up an API baseline automatically, so I've left the manual steps on the wiki.

- I've uploaded an oomph configuration to the ui git repository, but the main oomph repository isn't linking to ours yet.

The old manual installation steps should still work, but I've removed them from the wiki to avoid confusion.

  - Stefan