Add CONTRIBUTING.md

[eirizarry]

Create CONTRIBUTING.md that outlines how to contribute to the WorldWind Java repository. This will also update the repository's README.md

The new CONTRIBUTING.md will contain the following instructions for:

Asking a question
Reporting a bug
Suggesting enhancments
Adding an issue
Creating a pull request
Writing a commit message

These guidelines will be consistent with those found in all other WorldWind repositories.