update CHANGELOG

/cc @cbednarski - I try to do this after every PR just so I don't
forget, not a bad practice to get into. This is perhaps arguably an
"improvement" but I think due to the magnitude of code change I think
its worth it in the features sectin. :)

CHANGELOG.md

@@ -42,6 +42,9 @@ FEATURES:
       batch files. [GH-2243]
   * **New provisioner: windows-restart**: Restart a Windows machines and
       wait for it to come back online. [GH-2243]
+  * **Compress post-processor supports multiple algorithms:** The compress
+      post-processor now supports lz4 compression and compresses gzip in
+      parallel for much faster throughput.
 
 IMPROVEMENTS: