Replaced manual Ubuntu installation instructions with a PPA and a note t...

The current Ubuntu installation instructions do not provide any kind of autostart facility and require keeping the code up to date manually, but don't even mention it.

I've replaced them with instructions for installing from my PPA and a note to use Debian instructions for building from source code (they are compatible with Ubuntu). You're welcome to sprinkle it with more disclaimers if you feel like it.

I believe the Debian instructions should be also expanded to include the "generate config file" and "find a friend" stages which are not currently mentioned by them. I'll follow up on that in a subsequent pull request.