Mention the periodic-update tradeoff

Trusting clients need to carefully consider whether or not perdiodic updates
are right for them. Mention a reason why peridodic updates might be good,
in addition to the already-mentioned reason why they might be bad.
Grammar
Link to IDL
Remove en from links.
Grammar
Add block send.
Fix up interface.
Sort
Symmetry for validation.
Reorg
Condense and comment.
Update README.md
Update README.md
Update README.md
Update README.md
Merge branch 'master' of https://github.com/libbitcoin/libbitcoin_protocol