Document HBOSS test cases known to fail under Null lock implementation

Signed-off-by: Sean Busbey <busbey@apache.org>

hbase-oss/README.md

@@ -43,6 +43,10 @@ below:
 The null implementation just provides no-op methods instead of actual locking
 operations. This functions as an easy way to verify that a test case has
 successfully reproduced a problem that is hidden by the other implementations.
+Notice this implementation should consistently fail tests enforcing atomicity,
+such as TestAtomicRename and TestCreateNonRecursive. Additional tests may
+eventually fail due to unpredictable race conditions, since no lock behaviour
+is being implemented.
 
 ### Local Implementation (org.apache.hadoop.hbase.oss.sync.LocalTreeLockManager)