Updated the project documentation in the package.

Sources/AmiiboService/Catalogs/AmiiboService.docc/Library.md

@@ -72,6 +72,31 @@ This leverages HTTP cache headers from the server and persists cached responses
 
 Alternatively, cache the results returned by ``AmiiboService`` directly in your application using any storage mechanism that fits your needs, such as an in-memory dictionary, a database, or a file-based store.
 
+## Testing
+
+The ``AmiiboClient`` protocol enables creating custom mock clients for testing, eliminating the need for network calls in unit tests. Conform to ``AmiiboClient`` and return stubbed data or throw ``AmiiboServiceError`` errors to verify your application's behavior:
+
+```swift
+import AmiiboService
+
+struct MyMockClient: AmiiboClient {
+    var error: AmiiboServiceError?
+
+    func getAmiibos(
+        by filter: AmiiboFilter
+    ) async throws(AmiiboServiceError) -> [Amiibo] {
+        if let error { throw error }
+        return []
+    }
+
+    // Implement remaining protocol requirements...
+}
+
+let service = AmiiboService(client: MyMockClient())
+```
+
+Inject the mock client into ``AmiiboService`` via its ``AmiiboService/init(client:)`` initializer to test how your code handles empty results, specific errors, or any other scenario without relying on the live backend.
+
 ## Tasks
 
 This library offers a set of ready-to-use tasks that simplify the interaction with the library, which the developer can use from any `Terminal` application.