Add PGP fingerprint to 020_installation.rst
Add PGP fingerprint to 020_installation.rst

I like the idea of verifying the integrity of applications, I download from the internet. So I was very happy to see that restic does provide SHA256-checksums which are signed with the maintainers PGP key.

The only thing I miss: I could not find a direct way to download the used PGP key and verify the keys fingerprint.

Doing some searches, I found:
* https://github.com/restic/rest-server/issues/121
* https://restic.net/blog/2015-09-16/verifying-code-archive-integrity/

To help other restic users, I think you should add information about your PGP key/fingerprint to this installation doc, too. To save you some precious time, I created a draft, how this doc might be expanded, in this pull-request. You are free to accept it or change the text to your liking.

I copied the key/fingerprint text from: ``restic/restic/master/doc/090_participating.rst``

Thank you for your work in restic!
restic/restic,restic/restic