Improve README.md

- Add more details about how to download the jQuery plugin.
- Add short notice about how to report potential issues about project.
Add CONTRIBUTING.md with initial instructions

Based on the example:
https://github.com/jzaefferer/jquery-validation/blob/master/CONTRIBUTING.md
Shorter main header for the CONTRIBUTE.md
Fix copyright date from README.md
Merge pull request #2 from d2s/documentation-improvements

Documentation improvements