core README tweak
async+perf: ch3, more on promise limitations
async+perf: ch3, finishing 'limitations' section
types+grammar: appendixA, fixing code typo, per @qgustavor
async+perf: starting ch4 'generators'
async+perf: ch4, starting 'breaking run-to-completion' section
Fixed a tiny typo
Merge pull request #212 from HugoGiraudel/patch-1

Fixed a tiny typo
async+perf: starting 'input + output' section
async+perf: ch4, more on 'iteration messaging' and 'tale of two questions'
async+perf: ch4, tweaks and clarifications
async+perf: ch4, discussion of iterators/iterables
async+perf: ch4, tweaks and clarifications to generators-as-iterators discussions
Typo fix: they've --> they're
Merge pull request #214 from tomsugden/patch-1

Typo fix: they've --> they're