Documented upgrade procedure
Documented upgrade procedure

Closes #1661

yacloud-io/teleport,yacloud-io/teleport,yacloud-io/teleport,gravitational/teleport,yacloud-io/teleport,yacloud-io/teleport,gravitational/teleport,yacloud-io/teleport,yacloud-io/teleport,gravitational/teleport,gravitational/teleport,gravitational/teleport,gravitational/teleport