Merge pull request #317281 from wolfgangwalther/postgresql-eol-policy

postgresql: add readme with eol-policy

pkgs/servers/sql/postgresql/default.nix

@@ -1,5 +1,13 @@
 self:
 let
+  # Before removing an EOL major version, make sure to check the versioning policy in:
+  # <nixpkgs>/nixos/modules/services/databases/postgresql.md
+  #
+  # Before removing, make sure to update it to the last minor version - and if only in
+  # an immediately preceding commit. This allows people relying on that old major version
+  # for a bit longer to still update up to this commit to at least get the latest minor
+  # version. In other words: Do not remove the second-to-last minor version from nixpkgs,
+  # yet. Update first.
   versions = {
     postgresql_12 = ./12.nix;
     postgresql_13 = ./13.nix;