feat: document api contract

tests/fixtures/pokeapi_mewtwo_species.json

@@ -0,0 +1,11 @@
+{
+  "name": "mewtwo",
+  "is_legendary": true,
+  "habitat": { "name": "rare" },
+  "flavor_text_entries": [
+    {
+      "flavor_text": "It was created by a scientist after years of horrific gene splicing and DNA engineering experiments.",
+      "language": { "name": "en" }
+    }
+  ]
+}