watchdog: add documentation for PM usage

Add the first rule we recently agreed on.
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>

Documentation/watchdog/watchdog-pm.txt

+The Linux WatchDog Timer Power Management Guide
+===============================================
+Last reviewed: 17-Dec-2018
+
+Wolfram Sang <wsa+renesas@sang-engineering.com>
+
+Introduction
+------------
+This document states rules about watchdog devices and their power management
+handling to ensure a uniform behaviour for Linux systems.
+
+
+Ping on resume
+--------------
+On resume, a watchdog timer shall be reset to its selected value to give
+userspace enough time to resume. [1] [2]
+
+[1] https://patchwork.kernel.org/patch/10252209/
+[2] https://patchwork.kernel.org/patch/10711625/