Update circular-dependency.md

was quite hard to find the issue here... Maybe it's good to add a warning about barrel files

Author: jakobwgnr

content/fundamentals/circular-dependency.md

@@ -6,6 +6,8 @@ While circular dependencies should be avoided where possible, you can't always d
 
 We also describe resolving circular dependencies between modules.
 
+> warning **Warning** A circular dependency might also be caused when using "barrel files"/index.ts files to group imports. Barrel files should be omitted when it comes to modules/providers. For more details please also see [this github issue](https://github.com/nestjs/nest/issues/1181#issuecomment-430197191). 
+
 #### Forward reference
 
 A **forward reference** allows Nest to reference classes which aren't yet defined using the `forwardRef()` utility function. For example, if `CatsService` and `CommonService` depend on each other, both sides of the relationship can use `@Inject()` and the `forwardRef()` utility to resolve the circular dependency. Otherwise Nest won't instantiate them because all of the essential metadata won't be available. Here's an example: