tdlib: 1.6.0 -> 1.6.9

fix hash

version 1.6.9 is not released, so use revision hash

add a comment to the rev about why we are not using `v${version}` anymore

pkgs/development/libraries/tdlib/default.nix

@@ -7,7 +7,13 @@ stdenv.mkDerivation rec {
   src = fetchFromGitHub {
     owner = "tdlib";
     repo = "td";
+
+    # At version 1.6.0, this line was `rev = "v${version}". Version 1.6.9 uses an explicit revision hash because 1.6.9
+    # is not a tdlib GitHub release, and is therefore not hosted at `https://github.com/tdlib/td/releases/tag/v1.6.9`.
+    # Please return to the `rev = "v${version}"` style on the next version bump if you can, since that will allow
+    # `nixpkgs-update` to update the package automatically.
     rev = "32f2338bd199dd06a1b4b5f1ad14f2d4f2868f01";
+
     sha256 = "0wv03hlgzrsc04kcwnwz6dsmkdzvhb0i1wjs08gzivwxw06pkq4n";
   };