DEVELOP.md: document how to kill ControlMaster sessions

Certus machines use ControlMaster by default.

Change-Id: I22994fdb3bcde4da471e25feb63bb8f70b748c3a

DEVELOP.md

@@ -105,7 +105,7 @@ It installs Go, Minikube, Tilt and any other dependencies required for Wormhole
     scripts/dev-setup.sh
 
 You then need to close and re-open your session to apply the new environment.
-If you use persistent SSH sessions, make sure to kill the session before reconnecting.
+If you use ControlMaster SSH sessions, make sure to kill the session before reconnecting (`ssh -O exit hostname`).
 
 Start a minikube session with recommended parameters: