In the last year `programs.oh-my-zsh` gained more complexity and since
the introduction of features like `customPkgs` which builds a
`ZSH_CUSTOM` path from a sequence of derivation a documentation may be
fairly helpful to make the knowledge how to use the module and how to
package new ZSH plugins visible.
See https://github.com/NixOS/nixpkgs/pull/43282#issuecomment-410770432
