Merge pull request #447 from underfin/useExisting-provider

docs(dependency-injection.md): add doc for `useExisting` provider

content/fundamentals/dependency-injection.md

@@ -122,6 +122,27 @@ export class ApplicationModule {}
 
 > info **Hint** If your factory needs other providers, you have to pass their tokens inside the `inject` array. Nest will pass instances as arguments of the function in the same order.
 
+#### Use existing 
+
+The `useExisting` allows you creating aliases for existing providers. For example, the token `AliasedLoggerService` is an alias for `LoggerService`.
+
+```typescript
+@Injectable()
+class LoggerService {}
+
+const loggerAliasProvider = {
+  provide: 'AliasedLoggerService',
+  useExisting: LoggerService
+};
+
+@Module({
+  providers: [LoggerService, loggerAliasProvider],
+})
+export class ApplicationModule {}
+```
+
+> info **Hint** The instance of `LoggerService` will be equal to the instance defined by `AliasedLoggerService` token.
+
 #### Export custom provider
 
 In order to export a custom provider, we can either use a token or a whole object. The following example shows a token case: