Describe a workaround for a safe-themes problem.

`(sml/apply-theme ‘automatic)` can attempt to apply a theme that hasn’t been used yet, and so hasn’t been marked safe. Worse, it can do this when launching emacs in `—daemon` mode, leading to **severe** debugging pain — the error message is obscure, it doesn’t provide a line number, `—debug-init` is hard to run when launching in `—daemon` mode, and the problem doesn’t manifest if you launch emacs in a window that loads a theme that you’ve already marked safe. I eventually solved this problem by inspired guesswork. I seek to save others from my fate.

This patch attempts to add a description of the problem and its workaround to the README.md. This is only one option, however. One could also:

- Decide that defaulting `sml/theme` to `automatic` is too unreliable and default it to, say, `respectful` instead. I’d make this argument, having invested two hours of my life that I will never get back.
- Write some code that, on initial install, cycles through all the themes and asks the user to mark each one safe, instead of doing this on-demand.

Thanks for your work on this package!