README: document patch_vbmeta_flag feature

README.md

@@ -49,6 +49,8 @@ __supported.patchlevels=__ will match against ro.build.version.security_patch fr
 
 `ramdisk_compression=auto` allows automatically repacking the ramdisk with the format detected during unpack. Changing `auto` to `gz`, `lzo`, `lzma`, `xz`, `bz2`, `lz4`, or `lz4-l` (for lz4 legacy) instead forces the repack as that format, and using `cpio` or `none` will (attempt to) force the repack as uncompressed.
 
+`patch_vbmeta_flag=auto` allows automatically using the default AVBv2 vbmeta flag on repack, and use the Magisk configuration Canary 23016+. Set to `0` forces keeping whatever is in the original AVBv2 flags, and set to `1` forces patching the flag (only necessary on few devices).
+
 `customdd="<arguments>"` may be added to allow specifying additional dd parameters for devices that need to hack their kernel directly into a large partition like mmcblk0, or force use of dd for flashing.
 
 `slot_select=active|inactive` may be added to allow specifying the target slot. If omitted the default remains `active`.