Add info on #: a special symbol for hsc files

[maurges]

This can cite the documentation
This should also mention the #{command args, args} syntax, it was this syntax that brought me here.
Even though it's not really haskell, hsc2hs is a tool that is transparently run by cabal, so someone might be confused by it (as I was).

[takenobu-hs]

@d86leader, I'll add this. Thanks!

[takenobu-hs]

Added, thanks.