Update README.md

Author: mikebronner

README.md

@@ -37,6 +37,7 @@ The following items currently do no work with this package:
 - caching of lazy-loaded relationships, see #127
 - using select() clauses in Eloquent queries, see #238 (work-around discussed in the issue)
 - using SoftDeletes on Models, see #237
+- using polymorphic relationships, see #239
 ```
 
 [![installation guide cover](https://user-images.githubusercontent.com/1791050/36356190-fc1982b2-14a2-11e8-85ed-06f8e3b57ae8.png)](https://vimeo.com/256318402)