Create a v1 => v2 migration guide

[coilysiren]

View the release notes for guidance on migrating => https://github.com/urfave/cli/releases/tag/v2.0.0

[coilysiren]

Here's a PR where @ffrank and @purpleidea make the v1 => v2 migration purpleidea/mgmt#571

[coilysiren]

Here's a quirky example of someone moving from the very very old v2 branch, to the latest v1 release sourcegraph/godockerize#10

☝️ I don't think that's the type of migration where's really looking to write documentation for? but it's interesting to look at

[coilysiren]

Here's another example migration PR google/cloud-print-connector#470