Add CHANGELOG entry for multi exec
## Features
+- Support multiple `--exec <cmd>` instances, see #406 and
+#960 (@tmccombs)
## Bugfixes
## Changes
+
- Changed `-u` flag to be equivalent to `-HI`. Multiple `-u` flags still allowed but do nothing, see #840 (@jacksontheel)
## Other
+
- Added installation instructions for RHEL8, see #989 (@ethsol)
# v8.3.2
