gpt: doc: README: Update README entry for gpt verify extension

./doc/README.gpt entry has been updated to explain usage of "gpt verify"
command.

Signed-off-by: Lukasz Majewski <l.majewski@majess.pl>
Reviewed-by: Tom Rini <trini@konsulko.com>

doc/README.gpt

@@ -168,9 +168,34 @@ To restore GUID partition table one needs to:
 
 2. Define 'CONFIG_EFI_PARTITION' and 'CONFIG_CMD_GPT'
 
-2. From u-boot prompt type:
+3. From u-boot prompt type:
    gpt write mmc 0 $partitions
 
+Checking (validating) GPT partitions in U-Boot:
+===============================================
+
+Procedure is the same as above. The only change is at point 3.
+
+At u-boot prompt one needs to write:
+   gpt verify mmc 0 [$partitions]
+
+where [$partitions] is an optional parameter.
+
+When it is not provided, only basic checks based on CRC32 calculation for GPT
+header and PTEs are performed.
+When provided, additionally partition data - name, size and starting
+offset (last two in LBA) - are compared with data defined in '$partitions'
+environment variable.
+
+After running this command, return code is set to 0 if no errors found in
+on non-volatile medium stored GPT.
+
+Following line can be used to assess if GPT verification has succeed:
+
+U-BOOT> gpt verify mmc 0 $partitions
+U-BOOT> if test $? = 0; then echo "GPT OK"; else echo "GPT ERR"; fi
+
+
 Partition type GUID:
 ====================