docs: add NixOS integration section

Just a very basic example of how to use the NixOS module that Nixpkgs
provides.

docs/integrations/nixos.rst

@@ -0,0 +1,33 @@
+NixOS
+=====
+
+Nixpkgs contains a `NixOS module to run PostgREST <https://search.nixos.org/options?channel=unstable&query=services.postgrest&type=options>`_, which can be enabled with ``services.postgrest.enable = true``.
+
+A PostgreSQL server can be enabled on the same machine with ``services.postgresql.enable = true``. Connections will use the name of the system user as user and database names by default, in this case ``postgrest``.
+
+A minimal example could look like this:
+
+.. code-block:: nix
+
+  {
+    pkgs,
+    ...
+  }:
+
+  {
+    services.postgresql = {
+      enable = true;
+      initialScript = pkgs.writeText "init.sql" ''
+        CREATE ROLE postgrest LOGIN NOINHERIT;
+        CREATE ROLE anon ROLE postgrest;
+      '';
+    };
+
+    services.postgrest = {
+      enable = true;
+      settings.db-anon-role = "anon";
+      settings.db-uri.dbname = "postgres";
+    };
+  }
+
+This will expose the PostgREST server on localhost on the NixOS machine and allow anonymous access.

docs/postgrest.dict

@@ -97,6 +97,7 @@ namespaced
 Nanos
 neq
 nginx
+NixOS
 nixpkgs
 npm
 nxl