Adds docs for the new API

docs/createGlobalState.md

@@ -1,6 +1,6 @@
 # `useGlobalState`
 
-A React hook which creates a globally shared state.
+A React hook that creates a globally shared state.
 
 ## Usage
 
@@ -30,3 +30,32 @@ const Demo: FC = () => {
   );
 };
 ```
+
+It also allows initializing the state with a function and using a function in the setState:
+
+```tsx
+const useGlobalValue = createGlobalState<number>(() => 0);
+
+const CompA: FC = () => {
+  const [value, setValue] = useGlobalValue();
+
+  return <button onClick={() => setValue(value => value + 1)}>+</button>;
+};
+
+const CompB: FC = () => {
+  const [value, setValue] = useGlobalValue();
+
+  return <button onClick={() => setValue(value => value - 1)}>-</button>;
+};
+
+const Demo: FC = () => {
+  const [value] = useGlobalValue();
+  return (
+    <div>
+      <p>{value}</p>
+      <CompA />
+      <CompB />
+    </div>
+  );
+};
+```