[otbn] Update the otbn-rig README.md
[otbn] Update the otbn-rig README.md

Embarrassingly, I'd forgotten that this document existed, so hadn't
updated it to match the various changes we've made to how the
generator works. Oops!

Signed-off-by: Rupert Swarbrick <290093e2ad2515ed2f14d749293c8cf8e02ca274@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan