Update CHANGELOG

CHANGELOG.md

@@ -9,6 +9,7 @@ IMPROVEMENTS:
 
 * core: builders can now give warnings during validation. warnings won't
   fail the build but may hint at potential future problems.
+* builder/digitalocean: Can now specify a droplet name
 * builder/virtualbox: Can now disable guest addition download entirely
   by setting "guest_additions_mode" to "disable" [GH-580]
 * builder/virtualbox,vmware: ISO urls can now be https [GH-587]