Documentation and IRC channel
The official Chivalry SDK documentation is available here, on our public documentation Confluence wiki
You can also check out Epic’s public UE3 documentation at the Unreal Developer Network, and peruse their UDK forums. Chivalry is based on the July 2012 QA approved build of UE3 (plus our own custom engine modifications), which is more-or-less equivalent to the July 2012 UDK.
If you have questions about things not covered by the documentation, you can ask here. You might also find answers if you search other UE3 mod or UDK-centric websites and communities. There are a lot of tutorials, guides, and YouTube videos for UDK development that are applicable for Chivalry modding.
If you want to chat in real-time with your fellow Chivalry modders, and possibly some devs, join us in #tornbanner on QuakeNet (webchat). I still recommend posting questions in this forum though; we don’t respond on IRC immediately – most of the team doesn’t check it at all – and posting to this forum means others can read answers.
(if you’re going to use the IRC channel often, you might want to use a client instead of webchat; I like HexChat)