chore(docs): Add docs on usage of custom router state serializer with…

… store freeze (#426)

docs/router-store/api.md

@@ -3,7 +3,9 @@
 ## Custom Router State Serializer
 
 During each navigation cycle, a `RouterNavigationAction` is dispatched with a snapshot of the state in its payload, the `RouterStateSnapshot`. The `RouterStateSnapshot` is a large complex structure, containing many pieces of information about the current state and what's rendered by the router. This can cause performance
-issues when used with the Store Devtools. In most cases, you may only need a piece of information from the `RouterStateSnapshot`. In order to pair down the `RouterStateSnapshot` provided during navigation, you provide a custom serializer for the snapshot to only return what you need to be added to the payload and store.
+issues when used with the Store Devtools. In most cases, you may only need a piece of information from the `RouterStateSnapshot`. In order to pare down the `RouterStateSnapshot` provided during navigation, you provide a custom serializer for the snapshot to only return what you need to be added to the payload and store.
+
+Additionally, the router state snapshot is a mutable object, which can cause issues when developing with [store freeze](https://github.com/brandonroberts/ngrx-store-freeze) to prevent direct state mutations. This can be avoided by using a custom serializer.
 
 To use the time-traveling debugging in the Devtools, you must return an object containing the `url` when using the `routerReducer`.
 
@@ -54,4 +56,4 @@ export const reducers: ActionReducerMap<State> = {
   ]
 })
 export class AppModule { }
-```
+```