.fd files

[K. Berry]

An old TODO file I had for the tds mentioned three things:

1) .fd and font .sty files
-- do we need to say something about them?  Probably.  But what?

2) a list of environment variables
-- would probably be out of date as soon as it was published?
   And it would be painful to list all of the dozens of relevant variables.

3) a description of file formats
-- This seems better as a separate document to me.  It's also a
   nontrivial undertaking, and doesn't seem essential to publish tds 1.0.


wdyt?