Update readme with a little warning

Author: bitops

README.md

@@ -4,3 +4,5 @@ swift-collections
 An attempt to implement a bunch of nice methods from Ruby's Enumerable module to give nicer collection manipulation syntax. 
 
 Inspired by [Collection-Each](https://github.com/oarrabi/Collection-Each)
+
+This is a work in progress and mainly for me to learn. If you would like to use these methods in your code, please feel free, but read the code first so you can see how naive the implementation is at the moment.