Add return values to SlateReact hooks documentation

docs/libraries/slate-react/hooks.md

@@ -2,30 +2,42 @@
 
 React hooks for Slate editors
 
-### `useFocused`
+### `useFocused(): boolean`
 
 Get the current `focused` state of the editor.
 
-### `useReadOnly`
+### `useReadOnly(): boolean`
 
 Get the current `readOnly` state of the editor.
 
-### `useSelected`
+### `useSelected(): boolean`
 
 Get the current `selected` state of an element.
 
-### `useSlate`
+### `useSlate(): ReactEditor`
 
 Get the current editor object from the React context. Re-renders the context whenever changes occur in the editor.
 
-### `useSlateWithV`
+### `useSlateWithV(): { editor: ReactEditor, v: number }`
 
 The same as `useSlate()` but includes a version counter which you can use to prevent re-renders.
 
-### `useSlateStatic`
+### `useSlateStatic(): ReactEditor`
 
 Get the current editor object from the React context. A version of useSlate that does not re-render the context. Previously called `useEditor`.
 
-### `useSlateSelection`
+### `useSlateSelection(): (BaseRange & { placeholder?: string | undefined; onPlaceholderResize?: ((node: HTMLElement | null) => void) | undefined }) | null`
 
 Get the current editor selection from the React context. Only re-renders when the selection changes.
+
+### `useSlateSelector<T>(selector: (editor: Editor): T, equalityFn?: (a: T, b: T) => boolean): T`
+
+Similar to `useSlateSelection` but uses redux style selectors to prevent rerendering on every keystroke.
+
+Bear in mind rerendering can only prevented if the returned value is a value type or for reference types (e.g. objects and arrays) add a custom equality function.
+
+Example:
+
+```typescript
+const isSelectionActive = useSlateSelector(editor => Boolean(editor.selection))
+```