Second draft - introduction

[steve donovan]

Hi all,

https://github.com/stevedonovan/lua-cookbook/blob/master/src/book/en_US/intro.md

Please don't regard this as cast-in-stone, but rather as raw material
that can be moved around and 'repurposed' to use a fashionable word.

It certainly feels too long, but I don't know the best places to cut it up.

The section on 'modules' is over-long and really should be split in
two. (It does not yet have a discussion of 'packages')

In the intro, variables are introduced informally, because the
global/local distinction is made later. So my plea is that there is
some pedagogical justification for breaking the golden rule, at least
just here.

I am looking for a good coroutine example which is actually useful...

steve d.