fix: update docs around pluralizedTypes and type the field

Author: SergeAstapov

README.md

@@ -22,6 +22,7 @@ import Resolver from 'ember-resolver';
 
 export default class AppResolver extends Resolver {
   pluralizedTypes = {
+    ...this.pluralizedTypes,
     'sheep': 'sheep',
     'strategy': 'strategies'
   }

addon/index.d.ts

@@ -1,4 +1,6 @@
 import { Resolver as ResolverContract } from "@ember/owner";
 import EmberObject from "@ember/object";
 export default class Resolver extends EmberObject {}
-export default interface Resolver extends Required<ResolverContract> {}
+export default interface Resolver extends Required<ResolverContract> {
+    pluralizedTypes: Record<string, string>;
+}