Add Editable and event handling docs for Slate React

docs/libraries/slate-react/event-handling.md

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+# Slate React Event Handling
+
+By default, the `Editable` component comes with a set of event handlers that handle typical rich-text editing behaviors (for example, it implements its own `onCopy`, `onPaste`, `onDrop`, and `onKeyDown` handlers).
+
+In some cases you may want to extend or override Slate's default behavior, which can be done by passing your own event handler(s) to the `Editable` component.
+
+Your custom event handler can control whether or not Slate should execute its own event handling for a given event after your handler runs depending on the return value of your event handler as described below.
+
+```jsx
+import {Editable} from 'slate-react';
+
+function MyEditor() {
+  const onClick = event => {
+    // Implement custom event logic...
+
+    // When no value is returned, Slate will execute its own event handler when
+    // neither isDefaultPrevented nor isPropagationStopped was set on the event
+  };
+
+  const onDrop = event => {
+    // Implement custom event logic...
+
+    // No matter the state of the event, treat it as being handled by returning
+    // true here, Slate will skip its own event handler
+    return true;
+  };
+
+  const onDragStart = event => {
+    // Implement custom event logic...
+
+    // No matter the status of the event, treat event as *not* being handled by
+    // returning false, Slate will execute its own event handler afterward
+    return false;
+  };
+
+  return (
+    <Editable
+      onClick={onClick}
+      onDrop={onDrop}
+      onDragStart={onDragStart}
+      {/*...*/}
+    />
+  )
+}
+```