bats: add usage example to longDescription

The `bats` package provides a `withLibraries` function to easily include helper library such as `bats-assert` and `bats-support`.
However, this capability is not immediately visible to users browsing packages via `Nixpkgs Package Search`.

This commit adds a `longDescription` to document this feature, providing a code example on how to compose `bats` with its libraries.

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

@@ -255,6 +255,21 @@ 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:
+
+        bats.withLibraries (p: [
+          p.bats-assert
+          p.bats-support
+        ])
+    '';
     mainProgram = "bats";
     maintainers = with lib.maintainers; [ abathur ];
     license = lib.licenses.mit;