Revert "Revert "doc: Update CapsuleUpdate READMEs""

This reverts commit a7e4f905.
The description originally written by Sughosh is still valid
even after the commit 47a25e81 ("Revert "efi_capsule: Move signature
from DTB to .rodata"") was applied.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>