watchdog: documentation: update index file

Add the convert_drivers_to_kernel_api.txt to the 00-INDEX file.
Signed-off-by: NWim Van Sebroeck <wim@iguana.be>

Documentation/watchdog/00-INDEX

 00-INDEX
 	- this file.
+convert_drivers_to_kernel_api.txt
+	- how-to for converting old watchdog drivers to the new kernel API.
 hpwdt.txt
 	- information on the HP iLO2 NMI watchdog
 pcwd-watchdog.txt