docs: Add referential equality to pitfalls (#731)

Add a section about referential equality to pitfalls, with the simple 
example of using `indexOf` on a draft array to match an element.

Fixes #730

Author: ianstormtaylor

docs/pitfalls.md

@@ -70,3 +70,22 @@ produce(state, draft => {
 	})
 })
 ```
+
+### Drafts aren't referentially equal
+
+Draft objects in Immer are wrapped in `Proxy`, so you cannot use `==` or `===` to test equality between an original object and its equivalent draft (eg. when matching a specific element in an array). Instead, you can use the `original` helper:
+
+```javascript
+const remove = produce((list, element) => {
+  const index = list.indexOf(element) // this won't work!
+  const index = original(list).indexOf(element) // do this instead
+  if (index > -1) list.splice(index, 1)
+})
+
+const values = [a, b, c]
+remove(values, a)
+```
+
+If possible, it's recommended to perform the comparison outside the `produce` function, or to use a unique identifier property like `.id` instead, to avoid needing to use `original`.
+
+