crypto/openssl: update vendor update instructions

This change fills out the requirements for doing vendor updates,
documents the new vendor update process, and guides whoever needs to do
the next version update a bit better than the documentation did prior to
this change so everyone can pitch in with version updates a bit better.

Convert the document to Markdown while here to make it easier to
render/print out the directions in a structured format.

MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D53190

Conflicts:
crypto/openssl/FREEBSD-upgrade

(cherry picked from commit 08cdcff58acb2aec881e42c7f097d6492d864898)