You want to participate and contribute to the kernel development or integrate new MCU and platform support? You're welcome! Read the newcomer guide and subscribe to the RIOT-devel mailing list, which is the right place in case you have questions. For live discussions, join the IRC channel #riot-os at irc.freenode.net.
The RIOT issue tracker informs about bugs and enhancement requests. You could also subscribe to the notifications mailing list to get informed about new issues, comments, and pull requests. All commits to the source code will be posted to the commits mailing list.