Document upgrade/downgrade policy

[troglobit]

Somewhere prominently there should be some information about the general upgrade/downgrade policy. E.g.,

1. The guaranteed way to successfully upgrade -- do a factory reset
2. Syntax migration of startup-config after upgrade is supported
3. Downgrading is not supported
4. The guaranteed way to successfully downgrade, restore an backup of startup-config before calling the upgrade command

[jovatn]

Add test framework for upgrade migration as part of this task.