Added snap (snapcraft) generation file

[simos]

Snaps are installation packages similar to .deb, .rpm, flatpack, etc.
Snaps are used on Ubuntu and several other Linux distributions.

Here is the necessary file to generate a snap package for stacks-cli.
To generate the .snap installation package, install the snapcraft utility.
Then, run
snapcraft
The output is

Preparing to build stacks-cli 
Building stacks-cli 
Staging stacks-cli 
Priming stacks-cli 
Snapping 'stacks-cli' -                                                                                                                                                                              
Snapped stacks-cli_commit-29baf94_amd64.snap

You can then install the snap file as a developer with

snap install --devmode stacks-cli_commit-29baf94_amd64.snap

To publish the snap of stacks-cli, run the following:

snapcraft login      # Create an account if you do not have one, at https://login.launchpad.net/
snapcraft register stacks-cli    # Here you become the owner of the 'stacks-cli' name as snap package.
snapcraft push stacks-cli_commit-29baf94_amd64.snap   # Upload the snap to the store.
#   It takes a few seconds to get the app autoapproved (or see https://myapps.developer.ubuntu.com/ )
snapcraft release stacks-cli 1 stable    # Releases the snap package for anyone to install
snap install stacks-cli      # Here is how anyone can then install the utility.