docs: add breaking changes to v19 migration (#4620)

projects/ngrx.io/content/guide/migration/v19.md

@@ -21,4 +21,146 @@ Version 19 has the minimum version requirements:
 
 ## Breaking changes
 
-TODO
+### Signals
+
+#### Throw error in dev mode on state mutation
+
+The `patchState` method applies a deep freeze on the state in dev mode.
+If you try to update the state directly, it will throw an error in dev mode.
+
+BEFORE:
+
+```ts
+const userState = signalState(initialState);
+patchState(userState, (state) => {
+  // mutable change which went through
+  state.user.firstName = 'mutable change';
+  return state;
+});
+```
+
+AFTER:
+
+```ts
+const userState = signalState(initialState);
+patchState(userState, (state) => {
+  // mutable change throws in dev mode
+  state.user.firstName = 'mutable change'; 
+  return state;
+});
+```
+
+To fix the error, update the state in an immutable way.
+
+```ts
+const userState = signalState(initialState);
+patchState(userState, (state) => {
+  return {
+    ...state,
+    user: {
+      ...state.user,
+      // immutable change which went through
+      firstName: 'immutable change', 
+    },
+  };
+});
+```
+
+#### `computed` is replaced with `props`
+
+To support more cases, the `props` property is added to  `signalStoreFeature`, which replaces the existing `computed` property.
+This change has an ng-update schematic that automatically updates your code.
+
+- The `computed` property in the `SignalStoreFeature` type is renamed to `props`.
+- The `computed` property in the `signalStoreFeature` method is renamed to `props`.
+- The `EntityComputed` and `NamedEntityComputed` types in the `entities` plugin are renamed to `EntityProps` and `NamedEntityProps`.
+
+BEFORE:
+
+```ts
+import { computed, Signal } from '@angular/core';
+import {
+  signalStoreFeature,
+  SignalStoreFeature,
+  type,
+  withComputed,
+} from '@ngrx/signals';
+import { EntityComputed } from '@ngrx/signals/entities';
+
+export function withTotalEntities<Entity>(): SignalStoreFeature<
+  { state: {}, computed: EntityComputed<Entity>, methods: {} },
+  { state: {}, computed: { total: Signal<number> }, methods: {} },
+> {
+  return signalStoreFeature(
+    { computed: type<EntityComputed<Entity>>() },
+    withComputed(({ entities }) => ({
+      total: computed(() => entities().length),
+    })),
+  );
+}
+```
+
+AFTER:
+
+```ts
+import { computed, Signal } from '@angular/core';
+import {
+  signalStoreFeature,
+  SignalStoreFeature,
+  type,
+  withComputed,
+} from '@ngrx/signals';
+import { EntityProps } from '@ngrx/signals/entities';
+
+export function withTotalEntities<Entity>(): SignalStoreFeature<
+  { state: {}, props: EntityProps<Entity>, methods: {} },
+  { state: {}, props: { total: Signal<number> }, methods: {} },
+> {
+  return signalStoreFeature(
+    { props: type<EntityProps<Entity>>() },
+    withComputed(({ entities }) => ({
+      total: computed(() => entities().length),
+    })),
+  );
+}
+```
+
+#### Rename `rxMethod.unsubscribe()` to `rxMethod.destroy()`
+
+The `unsubscribe` method from `rxMethod` is renamed to `destroy`.
+
+BEFORE:
+
+```ts
+const logNumber = rxMethod<number>(tap(console.log));
+
+const num1Ref = logNumber(interval(1_000));
+const num2Ref = logNumber(interval(2_000));
+
+// destroy `num1Ref` after 2 seconds
+setTimeout(() => num1Ref.unsubscribe(), 2_000);
+
+// destroy all reactive method refs after 5 seconds
+setTimeout(() => logNumber.unsubscribe(), 5_000);
+```
+
+AFTER:
+
+```ts
+const logNumber = rxMethod<number>(tap(console.log));
+
+const num1Ref = logNumber(interval(1_000));
+const num2Ref = logNumber(interval(2_000));
+
+// destroy `num1Ref` after 2 seconds
+setTimeout(() => num1Ref.destroy(), 2_000);
+
+// destroy all reactive method refs after 5 seconds
+setTimeout(() => logNumber.destroy(), 5_000);
+```
+
+### Schematics
+
+#### Standalone is the default
+
+(Container) components created by the Container Schematic are now standalone by default.