update changelog for mm stuff

CHANGELOG.txt

 Changes in Release 1.5
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+  * Wrapper for OpenMM. This allows for direct access to molecular
+    mechanics functionality from within OST.
   * The compoundslib now features InChI and InChIKeys (machine readable InChI
     hash)
   * Names of compounds are also stored in the compoundslib now
 
+
 Changes in Release 1.4
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