add change log

Author: Xiangpeng Hao

CHANGELOG.md

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+# Change Log
+
+## Nov 2019
+In this update, we fixed several bugs and improved  performance.
+
+A big thank you to the following folks that helped to make PMwCAS even better.
+
+- Wojciech Golab and his students from the University of Waterloo found a linearizability bug, which may cause spurious abort in some cases.
+- Ziqi Wang from Carnegie Mellon University found an optimization that can improve the PMwCAS by reducing persistent memory flushes.
+- Shaonan Ma from Tsinghua University located a bug in PMDK integration that a crash during allocation may cause a memory leak.
+
+## June 2019
+
+Added PMDK backend, bug fixes, and general improvements on code base.
+
+Note that the PMDK integration assumes a Linux environment and might not work well on Windows machines.