Improved slate-simulator docs. (#1972)

* Improved slate-simulator docs.

* Update index.md

docs/reference/slate-simulator/index.md

@@ -4,7 +4,7 @@
 import Simulator from 'slate-simulator'
 ```
 
-A simulator to help writing tests for Slate editors and plugins. By default the simulator does not include the core plugins as they have a lot of dependencies on browser-specific globals, so running them in CI environments is very hard. If you need the core plugins for your use case you need to import them manually.
+A simulator to help writing tests for Slate editors and plugins. By default the simulator does not include the [core plugins](https://docs.slatejs.org/guides/plugins#core-plugins) as they have a lot of dependencies on browser-specific globals, so running them in CI environments is very hard. If you need the core plugins for your use case you need to import them manually.
 
 ## Example
 
@@ -39,6 +39,46 @@ const plugins = [ BeforePlugin(), ... , AfterPlugin() ]
 const simulator = new Simulator({ value, plugins })
 ```
 
+Core plugins will trigger default behaviour for the events. Without them, only the changes defined in the plugins passed to slate-simulator will be applied. For example, `beforeInput()` event will not insert the data's text if none of the plugins being tested does it.
+
+## Example for `DataTransfer` events
+
+In order to simulate paste and drop events you will need to create a [DataTransfer](https://developer.mozilla.org/en-US/docs/Web/API/DataTransfer). However this object is browser dependant and is not easy to manipulate in CI environments. The easiest work around is to fake its API with a mockup class:
+
+```js
+class FakeDataTransfer {
+  constructor(props) {
+    this.items = []
+    this.types = []
+  }
+
+  getData(key) {
+    return this.items.find(item => item.key === key).value
+  }
+
+  setData(key, value) {
+    this.types.push(key)
+    this.items.push({ key, value })
+  }
+}
+```
+
+Later, you can use it this way to fake a paste event:
+
+```js
+import { setEventTransfer } from 'slate-react'
+
+const pastedText = 'slatejs.org'
+
+const fakeDataTransfer = new FakeDataTransfer()
+fakeDataTransfer.setData('text', "this text doesn't matter")
+
+const pasteEvent = { dataTransfer: fakeDataTransfer }
+setEventTransfer(pasteEvent, 'text', pastedText)
+
+simulator.paste(pasteEvent)
+```
+
 ## Methods
 
 ### `beforeInput`