The main source of information about installing and using NixOS is the NixOS manual. Inside a NixOS system, you can read the manual by running the command nixos-help, and you can see the list of configuration options by running man configuration.nix. You can also search the list of options online.
There is also a separate mailing list to keep track of security releases. To subscribe either visit the nix-security-announce Google Group or send an email to firstname.lastname@example.org.
If you think you’ve found a bug, please report it in the Nixpkgs issue tracker (please use the label 6.topic: nixos).
There are several other manuals: