Add snap installation instructions

[flexiondotorg]

This pull request adds installation instructions for users of snap enabled Linux distributions. Snaps enable ISVs and projects to make their software available to millions of Linux users via the snap store and directly control delivery of software updates to their users. Snaps of desktop applications will also be available via the desktop Software Center, improving the discoverability of your software. Your software can be installed with a simple snap install shout simplifying the installation instructions for your users.

[dgw]

quietly points at thelounge/lounge

[astorije]

When you run snap install shout, what does this install? There is no Snap build from this repo, so I'm curious where the build comes from.