[css-syntax] Added section on defining rule grammars

To address the problems with defining rule grammars (which currently
are forced to use token-base definitions which are hard to read and
write, and generally are just copy-pasted from other specs), I added a
section to Syntax dedicated to this:
<http://dev.w3.org/csswg/css-syntax/#defining-grammars>

It contains guidelines and requirements for writing rule grammars,
including the fact that the ending token may be omitted in some
circumstances.  It also defines three production for use in the
contents of blocks: <declaration-list>, <rule-list>, and <stylesheet>.

There are several examples of existing at-rules in the section and how
to define them, including @font-face, @keyframes, and @media.

I think that what I have now is sufficient for all at-rules, and would
like some review to make sure of this.

~TJ