Add mention of being a fork of redux-immutable-state-invariant

docs/api/getDefaultMiddleware.md

@@ -55,10 +55,12 @@ One of the goals of Redux Toolkit is to provide opinionated defaults and prevent
 `getDefaultMiddleware` includes some middleware that are added **in development builds of your app only** to
 provide runtime checks for two common issues:
 
-- [`immutable-state-invariant`](./otherExports.md#createimmutablestateinvariantmiddleware): - deeply compares
+- [`immutable-state-invariant`](./otherExports.md#createimmutablestateinvariantmiddleware): deeply compares
   state values for mutations. It can detect mutations in reducers during a dispatch, and also mutations that occur between
   dispatches (such as in a component or a selector). When a mutation is detected, it will throw an error and indicate the key
-  path for where the mutated value was detected in the state tree.
+  path for where the mutated value was detected in the state tree. 
+
+    Forked from [`redux-immutable-state-invariant`](https://github.com/leoasis/redux-immutable-state-invariant)
 - [`serializable-state-invariant-middleware`](./otherExports.md#createserializablestateinvariantmiddleware): a custom middleware created specifically for use in Redux Toolkit. Similar in
   concept to `immutable-state-invariant`, but deeply checks your state tree and your actions for non-serializable values
   such as functions, Promises, Symbols, and other non-plain-JS-data values. When a non-serializable value is detected, a