Add snap packaging and install instructions

[Nightmayr]

I've added a snapcraft.yaml, which builds a snap successfully allowing nvtop to be installed on any Linux distro with snapd installed and be more up to date (as well as its dependencies) than the version that is usually in the distribution repos. Testing the snap locally on my machine shows it working as expected.

To transfer the snap into your ownership you would need to:

1. create a developer account on Snapcraft.io
2. I would then add your account as a collaborator on the snap
3. Raise a request on the Snapcraft forum to transfer the snap to you (I can stay on as a collaborator if you wish)

You can then configure in the developer dashboard to have automated builds of the snap when a push is made to the default branch of this repo. The resulting snap will end up on the edge channel, from which it can be promoted to stable on the dashboard after manual testing.

The snap currently needs the hardware-observe interface to be connected for AMD GPU fan speeds to be visible and process-control to kill processes. I've raised an auto-connect request on the forum which should hopefully be approved, meaning that on install the snap won't need the user to manually enable the permission.