Issue 2264: update readme

https://code.google.com/p/otwarchive/issues/detail?id=2264

This is a documentation change, but since it's a very visible documentation change, please give feedback if anything seems missing or incorrect or too much information. :) The link to the footer is also updated to point to the same license (GPL v2).
I'd suggest something more like:

[Read more about volunteering with the OTW](http://transformativeworks.org/how-you-can-help/volunteer) or drop into [our Campfire chatroom](https://fanarchive.campfirenow.com/7e097) if you have any questions. You can also submit a pull request; we'll get back to you as soon as we can!