feat(effects): deprecate @effect decorator (#2855)

modules/effects/src/effect_decorator.ts

@@ -10,6 +10,10 @@ import { getSourceForInstance } from './utils';
 
 const METADATA_KEY = '__@ngrx/effects__';
 
+/**
+ * @deprecated The Effect decorator (`@Effect`) is deprecated in favor for the `createEffect` method.
+ * See the docs for more info {@link https://ngrx.io/guide/migration/v11#the-effect-decorator}
+ */
 export function Effect(config: EffectConfig = {}) {
   return function <T extends Object, K extends EffectPropertyKey<T>>(
     target: T,

projects/ngrx.io/content/guide/migration/v11.md

@@ -126,3 +126,34 @@ router state:
 }
 */
 ```
+
+## Deprecations
+
+### @ngrx/effects
+
+#### The Effect decorator
+
+The Effect decorator, `@Effect`, is deprecated in favor for the `createEffect` method.
+
+See the [docs](/guide/effects#writing-effects) for more info.
+
+BEFORE:
+
+```ts
+@Effect()
+login$ = this.actions$.pipe(...);
+```
+
+AFTER:
+
+```ts
+login$ = createEffect(() => {
+  return this.actions$.pipe(...);
+});
+```
+
+To automatically migrate `@Effect` usages to the `createEffect` method, run the following NgRx migration:
+
+```sh
+ng generate @ngrx/schematics:create-effect-migration
+```