Split README to separate sections under doc/

The README was starting to get hard to parse with all the information.

This MR tries to split things up in sections under doc/.

assigned to @marcia

@marcia I need this to be merged before I tackle https://gitlab.com/gitlab-com/www-gitlab-com/issues/1486

mentioned in issue #1486 (closed)

I wonder why all this info isn't in the handbook instead, but fine either way. ;)

closed

reopened

oops, wrong button

merged

mentioned in commit 36acfc93

I wonder why all this info isn't in the handbook instead, but fine either way. ;)

@marcia yeah, we've been debating about this in https://gitlab.slack.com/archives/C0NFPSFA8/p1498038915517011.