Update README.md with Debian installation instructions

[mslinn]

No description provided.

[mislav]

Sorry, but this references a ppa maintained by someone else (cpick) who is not a maintainer of this project. Should cpick stop updating the ppa in the future, these instructions would become obsolete.

I would rather create a ppa under our control and keep it up to date after every official release. I've asked how to do that in #718 (comment) and if anyone has ideas how to set that up, please write there. Thanks!

[cpick]

@mislav for what it's worth, I recorded the steps I take every time I update my PPA here:
#718 (comment)

[mslinn]

I see my earlier comment has been deleted/censored. Uncool.

[mislav]

@mslinn I don't think anyone would delete your comment. I didn't receive any email notification for a comment that was later deleted. Was it in this thread, and if so, was it the text of the originally opened PR that is now gone, or a comment you've submitted later on?

@cpick Yes thank you for that. I will study that, but for now, the problem might be that I can't automate that via CI, since I can't spawn virtual machines via Vagrant within Travis CI environment. However, I could create a setup for publishing those PPA from my own machine.

[cpick]

@mislav I only use Vagrant to start from a fresh install and ensure I haven't forgotten to list some dependency. Really the process could skip all the Vagrant steps and still work just the same. (There are also other tools that attempt to accomplish the same thing, eg pbuilder, I'm just less familiar with them.) Most CI environments provide enough isolation between runs that they likely don't need any additional sandboxing.