README: Add Roadmap, update troubleshooting

* Updated Troubleshooting with real-world troubleshooting help
* Adding Roadmap / TODO in README.md:
  * Putting plans into shared artifacts helps people get on the same page

@sarina, I was talking to @stvstnfrd the other day and mentioned that the only reason certs aren't embarassingly parallel is because of their reliance on xqueue. He said that belonged in the README so it was sufficiently in-your-face that somebody might eventually do something about it. I think that that is a really good idea.

FWIW: I ran this with

```shell
LC_ALL=en_US.UTF8 python ./create_pdfs.py
```