clanServices: Add missing manifest.readme to some services

clanServices/matrix-synapse/README.md

@@ -0,0 +1,23 @@
+This NixOS module installs and configures Synapse — a federated Matrix homeserver with end-to-end encryption — and optionally provides the Element web client.
+
+The example below demonstrates a minimal setup that includes:
+
+- Element web client.
+- Synapse backed by PostgreSQL and nginx.
+- An admin user and an additional regular user.
+
+Example configuration:
+
+```nix
+instances = {
+    matrix-synapse = {
+        roles.default.machines."jon".settings = {
+            acmeEmail = "admins@clan.lol";
+            server_tld = "clan.test";
+            app_domain = "matrix.clan.test";
+            users.admin.admin = true;
+            users.someuser = { };
+        };
+    };
+};
+```