docs(store): add section for provideMockStore to testing guide (#1591)

projects/ngrx.io/content/guide/store/testing.md

@@ -1,6 +1,65 @@
 # Testing
 
-### Providing Store for testing
+### Using a Mock Store
+
+The `provideMockStore()` function registers providers that allow you to mock out the `Store` for testing functionality that has a dependency on `Store` without setting up reducers. 
+You can write tests validating behaviors corresponding to the specific state snapshot easily.
+
+<div class="alert is-helpful">
+
+**Note:** All dispatched actions don't affect to the state, but you can see them in the `Actions` stream.
+
+</div>
+
+Usage: 
+
+<code-example header="auth.guard.spec.ts">
+import { TestBed } from '@angular/core/testing';
+import { Store } from '@ngrx/store';
+import { provideMockStore } from '@ngrx/store/testing';
+import { cold } from 'jasmine-marbles';
+
+import { AuthGuard } from '../guards/auth.guard';
+import * as AuthActions from '../actions/auth-actions';
+
+describe('Auth Guard', () => {
+  let guard: AuthGuard;
+  let store: MockStore&lt;{ loggedIn: boolean }&gt;;
+  const initialState = { loggedIn: false };
+
+  beforeEach(() => {
+    TestBed.configureTestingModule({
+      imports: [
+        // any modules needed
+      ],
+      providers: [
+        AuthGuard,
+        provideMockStore({ initialState }),
+        // other providers
+      ],
+    });
+
+    guard = TestBed.get(AuthGuard);
+    store = TestBed.get(Store);
+  });
+
+  it('should return false if the user state is not logged in', () => {
+    const expected = cold('(a|)', { a: false });
+
+    expect(guard.canActivate()).toBeObservable(expected);
+  });
+
+  it('should return true if the user state is logged in', () => {
+    store.setState({ loggedIn: true });
+
+    const expected = cold('(a|)', { a: true });
+
+    expect(guard.canActivate()).toBeObservable(expected);
+  });
+});
+</code-example>
+
+### Using Store for Integration Testing
 
 Use the `StoreModule.forRoot` in your `TestBed` configuration when testing components or services that inject `Store`.