facetimehd: Document updating the facetimehd-firmware

pkgs/os-specific/linux/facetimehd/default.nix

@@ -11,6 +11,16 @@ stdenv.mkDerivation rec {
   src = fetchFromGitHub {
     owner = "patjak";
     repo = "bcwc_pcie";
+    # Note: When updating this revision:
+    # 1. Also update pkgs/os-specific/linux/firmware/facetimehd-firmware/
+    # 2. Test the module and firmware change via:
+    #    a. Give some applications a try (Skype, Hangouts, Cheese, etc.)
+    #    b. Run: journalctl -f
+    #    c. Then close the lid
+    #    d. Then open the lid (and maybe press a key to wake it up)
+    #    e. see if the module loads back (apps using the camera won't
+    #       recover and will have to be restarted) and the camera
+    #       still works.
     rev = "5a7083bd98b38ef3bd223f7ee531d58f4fb0fe7c";
     sha256 = "0d455kajvn5xav9iilqy7s1qvsy4yb8vzjjxx7bvcgp7aj9ljvdp";
   };