Update release notes, refs #57

Author: adangel

ReleaseNotes.md

@@ -17,6 +17,7 @@ Eclipse Update Site:
 
 *   [#48](https://github.com/pmd/pmd-eclipse-plugin/issues/48): Upgrade to PMD 6.3.0
 *   [#52](https://github.com/pmd/pmd-eclipse-plugin/issues/52): Eclipse Internal Error - Out of Memory
+*   [#57](https://github.com/pmd/pmd-eclipse-plugin/pull/57): De-duplicate project ruleset when loading the project properties
 
 ### External Contributions