Add contributing guideline

[jipiboily]

It should include:

• add tests
• ensure it works on any Ruby project, Rails-based or not
• add a CHANGELOG (Add a CHANGELOG? #230) entry
• update the doc/wiki/README
• ...?

[senny]

maybe a note about updating docs?

[jipiboily]

Yup, good point! (added to the above list)