update changelog for GH-5726, GH-5811, and GH-5851

Author: Mahmood Ali

CHANGELOG.md

@@ -6,6 +6,9 @@ IMPROVEMENTS:
 
 BUG FIXES:
 
+* drivers: Fixed an issue preventing external driver plugins from launching executor process [[GH-5726](https://github.com/hashicorp/nomad/issues/5726)]
+* drivers/docker: Fixed a bug mounting relative paths on Windows [[GH-5811](https://github.com/hashicorp/nomad/issues/5811)]
+* drivers/exec: Upgraded libcontainer dependency to avoid zombie `runc:[1:CHILD]]` processes [[GH-5851](https://github.com/hashicorp/nomad/issues/5851)]
 * ui: Fixed ability to click sort arrow to change sort direction [[GH-5833](https://github.com/hashicorp/nomad/pull/5833)]
 
 ## 0.9.3 (June 12, 2019)