Added migration instructions to README

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | n/a
| Fixed tickets | #95 
| License       | MIT
| Doc PR        | n/a
it's just a rewrite from scratch. I think we've gone beyond "refactoring" :)