[otbn] An initial user guide that shows how to run the OTBN tooling
[otbn] An initial user guide that shows how to run the OTBN tooling

This is rather incomplete. For example, it doesn't explain things like
how to generate a binary blob and include that blob into an Ibex
image. But I don't think we've actually decided how best to do that
yet!

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

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