Explain how to simply write any trigger. (#598)

* Explain how to simply write any trigger.

Addresses #573.

This new requirement only changes hardware with writable types. Even
there it is trivial to implement as long as types 2 or 6 are supported,
and those types become selected when 0 is written.

* Fix typo.

Co-authored-by: Ernie Edgar <43148441+ernie-sifive@users.noreply.github.com>

Co-authored-by: Ernie Edgar <43148441+ernie-sifive@users.noreply.github.com>