Based on a suggestion made by @petertodd to the -devel mailing list and
the discussion in that thread by him and other participants.
* We've been using the term "consensus", but this commit introduces a
formal definition for it and "consensus rules" as part of the block
chain introduction.
* Describe that consensus rules may change and may happen when they
do: hard or soft forks.
* Describe how full nodes can detect hard or soft forks, then describe
how SPV clients can detect hard and soft forks using the more limited
information available to them.
