This patch adds a php-src directory structure introduction to the
contributing guidelines. Thanks to the discussion at [1] and PHP
manual.
[1] https://github.com/php/php-src/pull/3520
I was not at all guilted into this by the open source summit at php[tek]. Not
at all.
(such guilt)
This is based mostly on information already on our Web site, and is primarily a
set of links to other pages. Still, if we can encourage more contributors,
let's do it!
