Copy installation instructions from the Dangerzone wiki [1] into the
Dangerzone source. This has several benefits:
1. Devs can update installation instructions as part of a PR.
2. Users can see installation instructions for previous releases.
The last point is important, because we can update our instructions in
the main branch, without affecting the instructions a user follows from
the website (currently pointing to the Dangerzone Wiki).
Refs #240
[1]: https://github.com/freedomofpress/dangerzone/wiki/Installing-Dangerzone
