Add store signature

Not sure exactly what the syntax of these signatures are, but they look like TypeScript, so that's what I went with for the object literal too.

Author: trbrc

site/content/docs/01-component-format.md

@@ -174,7 +174,15 @@ Local variables (that do not represent store values) must *not* have a `$` prefi
 
 ---
 
-You can create your own stores without relying on [`svelte/store`](docs#svelte_store), by implementing the **store contract**:
+##### Store contract
+
+```js
+store = { subscribe: (subscription: (value: any) => void) => () => void, set?: (value: any) => void }
+```
+
+---
+
+You can create your own stores without relying on [`svelte/store`](docs#svelte_store), by implementing the *store contract*:
 
 1. A store must contain a `.subscribe` method, which must accept as its argument a subscription function. This subscription function must be immediately and synchronously called with the store's current value upon calling `.subscribe`. All of a store's active subscription functions must later be synchronously called whenever the store's value changes.
 2. The `.subscribe` method must return an unsubscribe function. Calling an unsubscribe function must stop its subscription, and its corresponding subscription function must not be called again by the store.