Summary of store contract in `svelte/store` docs

site/content/docs/03-run-time.md

@@ -214,7 +214,11 @@ Events dispatched from child components can be listened to in their parent. Any
 
 ### `svelte/store`
 
-The `svelte/store` module exports functions for creating [stores](docs#4_Prefix_stores_with_$_to_access_their_values).
+The `svelte/store` module exports functions for creating [readable](docs#readable), [writable](docs#writable) and [derived](docs#derived) stores.
+
+Keep in mind that you don't *have* to use these functions to enjoy the [reactive `$store` syntax](docs#4_Prefix_stores_with_$_to_access_their_values) in your components. Any object that correctly implements `subscribe`/`unsubscribe` and (optionally) `set` is a valid store, and will work both with the special syntax, and with Svelte's built in [`derived` stores](docs#derived).
+
+This makes it possible to wrap almost any other reactive state handling library for use in Svelte. Read more about the [store contract](docs#4_Prefix_stores_with_$_to_access_their_values) to see what a correct implementation looks like.
 
 #### `writable`