var vs. ro data in TDS

[K. Berry]

I asked a friend to look at the draft TDS, and one of his immediate
comments was:

    2. The Linux File System Standard has texmf directories in two places:
    one for invariant data (which can then be in a read-only filesystem) and
    one (/var/lib/texmf) for variable data such as dynamically constructed
    fonts.  I think it's important that you accommodate this standard.

We can't require any such thing, I think, but we should probably mention
this. I guess it's handled as a separate ``local'' tree or something?
Any thoughts?