Merge pull request #1665 from bot101/patch-1

Update unions.md - Added information that let's the user know to import the resolver in a schema-first approach

Author: kamilmysliwiec

content/graphql/unions.md

@@ -114,7 +114,7 @@ export class Book {
 export type ResultUnion = Author | Book;
 ```
 
-Unions require an extra `__resolveType` field in the resolver map to determine which type the union should resolve to. Let's create a `ResultUnionResolver` class and define the `__resolveType` method:
+Unions require an extra `__resolveType` field in the resolver map to determine which type the union should resolve to. Also, note that the `ResultUnionResolver` class has to be registered as a provider in any module. Let's create a `ResultUnionResolver` class and define the `__resolveType` method. 
 
 ```typescript
 @Resolver('ResultUnion')