bats: add usage example to longDescription (#478728)

pkgs/by-name/ba/bats/package.nix

@@ -255,6 +255,22 @@ resholve.mkDerivation rec {
   meta = {
     homepage = "https://github.com/bats-core/bats-core";
     description = "Bash Automated Testing System";
+    longDescription = ''
+      Bats can be extended with libraries. The available libraries are:
+
+      - `bats-assert`
+      - `bats-file`
+      - `bats-detik`
+      - `bats-support`
+
+      An example of building this package with a few libraries:
+      ```nix
+      bats.withLibraries (p: [
+        p.bats-assert
+        p.bats-support
+      ])
+      ```
+    '';
     mainProgram = "bats";
     maintainers = with lib.maintainers; [ abathur ];
     license = lib.licenses.mit;