Add documentation

libs/ui/src/lib/entity-logo/entity-logo-image-source.service.ts

@@ -2,6 +2,7 @@ import { Injectable } from '@angular/core';
 import { DataSource } from '@prisma/client';
 
 @Injectable({
+  // Required to allow mocking in Storybook
   providedIn: 'root'
 })
 export class EntityLogoImageSourceService {