Merge pull request #29 from fdegiuli/memory-docs

docs: Update memory backend docs

Author: jkrems

README.md

@@ -91,6 +91,7 @@ cached('myStuff', { backend: {
 ### Memory
 
 Stores all the data in an in-memory object. 
+*__Caveat:__ `get()` will return a reference to the stored value. Mutating the returned value will affect the value in the cache.*
 
 #### Example