Current documentation for buildroot includes:

Buildroot - Usage and documentation is where you want to start reading if you wish to understand how buildroot work, or wish to change/extend/fix things. If you find any errors (factual, grammatical, whatever) please report them.
README. This is the README file included in the buildroot source release.
If you find that you need help with buildroot, you can ask for help on the buildroot mailing list at buildroot at uclibc.org. In addition the BusyBox, uClibc, and buildroot developers are also known to hang out on the uClibc IRC channel: #uclibc on irc.freenode.net.