Add a readOnly function to svelte/store

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

@@ -365,6 +365,38 @@ const delayed = derived([a, b], ([$a, $b], set) => {
 });
 ```
 
+
+#### `readOnly`
+
+```js
+readable = readOnly(writable: Writable<T>)
+```
+
+---
+
+Sometimes readable and derived aren't the ideal tools for more complex custom stores. For stores that need to only be writable in their module, and _not_ outside, you can use `readOnly` to get a readonly version of a writable store.
+
+```js
+import { writable, readOnly } from 'svelte/store';
+
+const userStore = writable({});
+
+export function logIn(username, password) {
+	if (password == 'password') {
+		userStore.set({ username, error: null });
+	} else {
+		userStore.set({ username: null, error: 'Bad Password' });
+	}
+}
+
+export function logOut() {
+	userStore.set({});
+}
+
+export const user = readOnly(userStore);
+```
+
+
 #### `get`
 
 ```js

src/runtime/store/index.ts

@@ -196,6 +196,17 @@ export function derived<T>(stores: Stores, fn: Function, initial_value?: T): Rea
 	});
 }
 
+/**
+ * Get a readable store from a writable store.
+ *
+ * @param store writable
+ *
+ * @returns readable store
+ */
+export function readOnly<T>(store: Writable<T>): Readable<T> {
+	return { subscribe: store.subscribe };
+}
+
 /**
  * Get the current value from a store by subscribing and immediately unsubscribing.
  * @param store readable