Initial coding conventions doc

Summary: This is mostly the uncontroversial stuff that we already agree on. I'd
like to go after the remaining stuff like switch statement formatting too but
that can come in later diffs after much discussion.

Reviewed By: @edwinsmith

Differential Revision: D1394638
Expand jit documentation a bit

Summary: I added more about the hhbc -> ir transformation, hhir itself, and
some optimizations.

Reviewed By: @edwinsmith

Differential Revision: D1397404