README: update requirements for signed dm-verity

The newest kconfig enabling DB-verified dm-verity images is queued
for 6.11:

https://patchwork.kernel.org/project/dm-devel/patch/[email protected]/
(cherry picked from commit a79b6dc)
(cherry picked from commit c32f71a)
(cherry picked from commit d34f0de)

Author: bluca

README

@@ -128,9 +128,10 @@ REQUIREMENTS:
 
         Required for signed Verity images support:
           CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG
-        Required to verify signed Verity images using keys enrolled in the MoK
-        (Machine-Owner Key) keyring:
+        Required to verify signed Verity images using keys enrolled in the MOK
+        (Machine-Owner Key) and DB UEFI certificate stores:
           CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG_SECONDARY_KEYRING
+          CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG_PLATFORM_KEYRING
           CONFIG_IMA_ARCH_POLICY
           CONFIG_INTEGRITY_MACHINE_KEYRING