Add README for FactoryReset interface

This commit adds a README explanation of the FactoryReset interface,
both in generic terms and a brief description of each of the current
implementations. This is intended to clarify what does and doesn't
occur when each factory reset implementation is called.

Resolves openbmc/openbmc#2229

Change-Id: Ic5072346dbe9bb87af05192de19ed17be2bb1482
Signed-off-by: Michael Tritz <mtritz@us.ibm.com>
diff --git a/xyz/openbmc_project/Common/FactoryReset/README.md b/xyz/openbmc_project/Common/FactoryReset/README.md
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+# Factory Reset
+
+## Overview
+
+The OpenBMC API defines a factory reset interface, which is intended to be used
+to restore the BMC to its original manufacturer settings. This interface is
+defined generically; it is specifically and variously implemented throughout
+OpenBMC services, which allows these services to be individually restored to
+factory defaults as needed.
+
+## Known Implementations (listed by D-Bus service)
+
+### xyz.openbmc_project.Network
+Path: `/xyz/openbmc_project/network`
+The network factory reset overwrites the configuration for all configured
+network interfaces to a DHCP setting. Configuration changes will take effect
+the next time each interface is brought up - either manually or during a BMC
+reboot.
+
+### xyz.openbmc_project.Software.BMC.Updater
+Path: `/xyz/openbmc_project/software`
+The BMC software updater factory reset clears any volumes and persistence files 
+created by the BMC processes. This reset occurs only on the next BMC reboot.