add doc/ppp-on-up.md

doc/ppp-on-up.md

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+Run scripts on ppp connection
+=============================
+
+[◀ Go back to main README](../README.md)
+
+Description
+-----------
+
+This script is supposed to run on established ppp connection. Currently
+it does:
+
+* release IPv6 dhcp leases (and thus force a renew)
+* run [update-tunnelbroker](update-tunnelbroker.md)
+
+Requirements and installation
+-----------------------------
+
+Just install the script:
+
+    $ScriptInstallUpdate ppp-on-up;
+
+... and make it the `on-up` script for ppp profile:
+
+    / ppp profile set on-up=ppp-on-up [ find ];
+
+See also
+--------
+
+* [Update configuration on IPv6 prefix change](ipv6-update.md)
+* [Update tunnelbroker configuration](update-tunnelbroker.md)
+
+---
+[◀ Go back to main README](../README.md)  
+[▲ Go back to top](#top)