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.
Thanks also (in alphabetical order) to @cbeams, @mikehearn, and
@tgeller, among others.
The last pre-squash commit was: c2b8d562aa107c7b68c60946cea14cdccc5159ad
