Mike Gunderloy has written an excellent thirteen step guide to contributing to Rails - step by step. He covers:

Getting Git
Checking out the Rails source code
Setting up and running the tests that come with Rails (so you can check that your code doesn't bust Rails)
Forking Rails (for doing your changes)
Updating Rails with the changes in your [...]

Hot off the back of the Rails Guides hackfest came a lot of great new Rails documentation. Now joining that documentation comes an all new Ruby on Rails Security Guide. Clocking in at almost 11,000 words, the guide covers RJS injection, cookie store session replay attacks, session hijacking, File upload security, mass assignment of attributes, [...]

It seems that some rather good writers have been very busy, because as if from no-where, a whole batch of awesome new Rails manuals (seemingly up to Rails 2.1 standards) have made an appearance at http://guides.rails.info/
There are no credits, but the content is all of a high quality. Including:

Getting Started With Rails - a walkthrough [...]

Outside of thread safety and internationalization support (both of which will be supported in Rails 2.2), one of the most maligned areas for Rails development has been the online documentation.
An effort to improve that situation though was recently posted on the Rails Weblog with the announcement of a Rails Guide Hackfest. For the Hackfest, they're [...]

Sven Fuchs is part of the team working on internationalization in the forthcoming Rails 2.2. Work began in September 2007 with the team deciding that enough was enough with the sloppy support for internationalization in Rails. They wanted to "eliminate the need for monkey patching Rails in order to internationalize an application" by "implementing a [...]

Rails-doc.org is a new attempt at organizing Rails' documentation by a Finnish company called Nodeta. As well as providing a quick way to search and browse through the existing documentation (with live search and keyboard accessible navigation), Rails-doc also allows registered members to annotate and leave notes against entries. Rails-doc.org's aim at improving Rails' documentation [...]