docs(store): add selector deprecation in v12 migration (ngrx#3015)

Author: timdeschryver

projects/ngrx.io/content/guide/migration/v12.md

@@ -19,6 +19,48 @@ Version 12 has the minimum version requirements:
 - TypeScript version 4.2.x
 - RxJS version 6.5.x
 
-## Breaking changes
+## Deprecations
 
-TBD
+### @ngrx/store
+
+#### Selectors With Props
+
+Selectors with props are deprecated in favor of "normal" factory selectors.
+Factory selectors have the following benefits:
+
+- easier to write and to teach
+- selectors are typed
+- child selectors are correctly memoized
+
+BEFORE:
+
+```ts
+const selectCustomer = createSelector(
+    selectCustomers,
+    (customers, props: { customerId: number }) => {
+      return customers[customerId];
+    }
+);
+
+// Or if the selector is already defined as a factory selector
+
+const selectCustomer = () =>
+  createSelector(
+    selectCustomers,
+    (customers, props: { customerId: number }) => {
+      return customers[customerId];
+    }
+  );
+​```
+
+AFTER:
+
+```ts
+const selectCustomer = (customerId: number) =>
+  createSelector(
+    selectCustomers,
+    (customers) => {
+      return customers[customerId];
+    }
+  );
+```