Link to auto-generated current rules

\cc @TiarkRompf @odersky

I took a first stab at generating the current rules from the Twelf development. The result is here:
http://lampwww.epfl.ch/~amin/dot/twelf.pdf

Lots of noise there, but the crux, subtyping, is pretty readable.

If you want to edit how a Twelf relation is rendered, renew its command in this latex file: src/dot_custom.tex