Document custom MELPA package build expressions

[psionic-k]

There are easier ways, but I found this in a stash

              # Manual definition for non-melpa package
              (epkgs.melpaBuild {
                pname = "melpazoid";
                version = "0.0.0";
                commit = "c7cc28607236aba92949e7c47ffb5affb03d6111";
                packageRequires = [
                  epkgs.pkg-info
                  epkgs.epl
                ];
                src = pkgs.fetchFromGitHub {
                  owner = "riscy";
                  repo = "melpazoid";
                  rev = "c7cc28607236aba92949e7c47ffb5affb03d6111";
                  sha256 = "sha256-CY3iwY6uKlMZFLEpgzQ6hNqSBpFUG+/IdF3mSBzST0s=";
                };
                recipe = pkgs.writeText "recipe" ''
                      (melpazoid
                       :repo "riscy/melpazoid"
                       :files ("melpazoid/melpazoid.el")
                       :fetcher github)
                     '';})