docs(frp): note ExApps need reinstall after cert regeneration

Signed-off-by: Oleksander Piskun <oleksandr2088@icloud.com>

Author: oleksandr-nc

README.md

@@ -403,6 +403,9 @@ The FRP client-server connections, i.e. the connection from the above FRP client
 > not renewed automatically. The FRP connection uses mutual TLS, so once a certificate expires the tunnel stops
 > working. To renew them, stop HaRP, delete its `/certs/frp` folder, and start HaRP again, then re-copy `client.crt`,
 > `client.key`, and `ca.crt` to each external Docker Engine and restart its `frpc`.
+>
+> ExApps deployed through HaRP embed these certificates at install time, so after regenerating them you must
+> **remove and re-install each ExApp** for it to pick up the new certificates (a restart is not enough).
 
 ## Adapting ExApps to use HaRP