chore(docs): Added payload interface options to migration docs

Closes #303

MIGRATION.md

@@ -43,7 +43,9 @@ import { compose } from '@ngrx/store';
 
 ### Action interface
 
-The `payload` property has been removed from the `Action` interface.
+The `payload` property has been removed from the `Action` interface. It was a source of type-safety
+issues, especially when used with `@ngrx/effects`. If your interface/class has a payload, you need to provide
+the type.
 
 BEFORE:
 ```ts
@@ -79,6 +81,22 @@ export class MyEffects {
 }
 ```
 
+If you prefer to keep the `payload` interface property, you can provide your own parameterized version.
+
+```ts
+export interface ActionWithPayload<T> extends Action {
+  payload: T;
+}
+```
+
+And if you need an unsafe version to help with transition.
+
+```ts
+export interface UnsafeAction implements Action {
+  payload?: any;
+}
+```
+
 ### Registering Reducers
 
 Previously to be AOT compatible, it was required to pass a function to the `provideStore` method to compose the reducers into one root reducer. The `initialState` was also provided to the method as an object in the second argument.