add instructions how to convert from ext2 to ext3 and ext4

mmdebstrap

@@ -5319,7 +5319,9 @@ C<genext2fs> utility, which will create an ext2 image that will be
 approximately 90% full in I<TARGET>.  The special I<TARGET> C<-> does not work
 with this format because C<genext2fs> can only write to a regular file. If you
 need your ext2 image be named C<->, then just explicitly pass the relative path
-to it like F<./->.
+to it like F<./->. To convert the result to an ext3 image, use C<tune2fs -O
+has_journal TARGET> and to convert it to ext4, use C<tune2fs -O
+extents,uninit_bg,dir_index,has_journal TARGET>
 
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