update changelog

Author: iSchluff

CHANGELOG.md

@@ -31,6 +31,7 @@
 - Add configuration option to allow Tailscale clients to use a random WireGuard port. [kb/1181/firewalls](https://tailscale.com/kb/1181/firewalls) [#624](https://github.com/juanfont/headscale/pull/624)
 - Improve obtuse UX regarding missing configuration (`ephemeral_node_inactivity_timeout` not set) [#639](https://github.com/juanfont/headscale/pull/639)
 - Fix nodes being shown as 'offline' in `tailscale status` [648](https://github.com/juanfont/headscale/pull/648)
+- Improve shutdown behaviour [651](https://github.com/juanfont/headscale/pull/651)
 
 ## 0.15.0 (2022-03-20)