Users need a guide for how to migrate from the Go Driver 1.x -> 2.0 API. Write a migration guide.

Open questions:

1. What is the best way to publish the migration guide? As a blog post? As a page in the Go driver docs site? Somewhere else?

Definition of done:

• Write a Go Driver BSON library 1.x -> 2.0 migration guide.
• Publish the migration guide somewhere (see open questions above) and link to it in the v2.0 release notes. Make sure it's the same place as GODRIVER-2705.