Adding immer in the list of immutable libraries (#1839)

* Adding immer in the list of immutable libraries

* Update optimizing-performance.md

content/docs/optimizing-performance.md

@@ -430,6 +430,6 @@ x === z; // true
 
 In this case, since a new reference is returned when mutating `x`, we can use a reference equality check `(x === y)` to verify that the new value stored in `y` is different than the original value stored in `x`.
 
-Two other libraries that can help use immutable data are [seamless-immutable](https://github.com/rtfeldman/seamless-immutable) and [immutability-helper](https://github.com/kolodny/immutability-helper).
+Other libraries that can help use immutable data include [Immer](https://github.com/mweststrate/immer), [immutability-helper](https://github.com/kolodny/immutability-helper), and [seamless-immutable](https://github.com/rtfeldman/seamless-immutable).
 
 Immutable data structures provide you with a cheap way to track changes on objects, which is all we need to implement `shouldComponentUpdate`. This can often provide you with a nice performance boost.