Write a "Getting Started" guide

[samuelkarp]

We need a guide to help users and contributors get started with firecracker-containerd. #16 is part of the process, but we also need to cover installation and configuration of all required components.

[IRCody]

Some things that I remember have come up that are not documented atm:

• Should use relative path as the socket_path inside. If this isn't done, because the socket isn't cleaned up cleanly every time. This results in behavior where you have roughly 50/50 success/error of starting firecracker. Relative path creates the socket inside the bundle dir which is unique per container.
• The cri plugin must currently be disabled for unknown reasons. I created Figure out why cri plugin must be disabled #35 to track this. This can be done by adding disabled_plugins = ["cri"] to the top of the /etc/containerd/config.toml file and restarting containerd.

[samuelkarp]

• Getting started
• Quickstart