[tex-live] install-tl.sh script comments

[Karl Berry]

    > I'm also not so sure if it is really needed ;-) And many asked me
    > why we have empty texmf-local structure installed by default.
    > Who knows?

I prominently mention and recommend texmf-local in the revised
documentation.

Having an empty tree for it means that users or tools don't have to
create directories, and can more easily see where to put files.

So I think it's a help, and I don't see that it causes any harm?