add short healthcheck doc

tailscale/DOCS.md

@@ -345,6 +345,17 @@ CGNAT networks). You can test connections with `tailscale ping
 
 When not set, an automatically selected port is used by default.
 
+## Healthcheck
+
+Tailscale is quite resilient and can recover from nearly any network change. In
+case it fails to recover, the add-on's health is set unhealthy. The add-on's
+health is checked by Home Assistant in each 30s, and if it reports itself 3
+times unhealthy in a row, the add-on will be restarted.
+
+The add-on's health is set unhealthy:
+- once it was online and gets offline for longer than 5 minutes
+- after a (re)start can't get online for longer than 1 hour
+
 ## Changelog & Releases
 
 This repository keeps a change log using [GitHub's releases][releases]