Add beginning of History docs to Slate docs

docs/Summary.md

@@ -54,6 +54,7 @@
 
 - [Slate React](libraries/slate-react.md)
 - [Slate History](libraries/slate-history.md)
+  - [History](/history/history.md)
 - [Slate Hyperscript](libraries/slate-hyperscript.md)
 
 ## General

docs/history/history.md

@@ -0,0 +1,19 @@
+# History
+
+The `History` object contains the undo and redo history for the editor.
+
+```typescript
+export interface History {
+  redos: Batch[]
+  undos: Batch[]
+}
+
+interface Batch {
+  operations: Operation[]
+  selectionBefore: Range | null
+}
+```
+
+It can be accessed from an `Editor` instance as the property `history`.
+
+This property is only available on the `Editor` if it was instantiated using the `withHistory` method which adds undo/redo functionality to the Slate editor.