Create a short guide on how to pin urfave/cli v1

[coilysiren]

This is conceptually related to #921

---

Frequently, people will opt for pinning v1 rather than migrating to v2. This is totally fine and something we generally are comfortable with - so long as we aren't flooded with "how do I pin v1" Github issues. In service of that, we should create a readme describing how to pin the latest v1 version.

Examples of PRs where people ping v1:

• Add go modules and update cli flag creation to work with latest urfave/cli changes anubhavmishra/envoy-consul-sds#4
• upgrade to github.com/urfave/cli@master sourcegraph/godockerize#10
• Guestlogix/pat@02d0481

[coilysiren]

Here's a blog post that's helpful => https://blog.golang.org/migrating-to-go-modules

[coilysiren]

I consider https://blog.golang.org/migrating-to-go-modules the best possible example here, so I'm closing this 👋