Update README.md (eugenp#1657)

Author: yetanotherallisonf

core-java/README.md

@@ -91,4 +91,4 @@
 - [Avoiding ConcurrentModificationException when iterating and removing](http://www.baeldung.com/avoiding-concurrentmodificationexception-when-iterating-and-removing)
 - [Removing all nulls from a List in Java](http://www.baeldung.com/java-remove-nulls-from-list)
 - [Removing all duplicates from a List in Java](http://www.baeldung.com/java-remove-duplicates-from-list)
-
+- [An Introduction to ThreadLocal in Java](http://www.baeldung.com/java-threadlocal)