Clarify differences between built-in memoize and core.memoize "replacement"
Due to differences in locking etc, there are some subtle differences in semantics between the clojure.core/memoize function and core.memoize/memo function, and these should be explained in the README.
README and docstring updated.
Confirmed that clojure.core/memoize can invoke the function multiple times for a given set of arguments when several concurrent calls occur but clojure.core.memoize/memo guards against that via locking. Will update the README (and the docstring) to clarify this.