migrate-to-v6.md

Migrate to v6

This guide references the major changes between v5 and v6 to help those migrating an existing app.

Removing SigningMethod and SigningMethodType

We moved these types to the api-kit.

// old v5 code
import { SigningMethod, SigningMethodType } from '@safe-global/protocol-kit'

// new v6 code
import { SigningMethod, SigningMethodType } from '@safe-global/types-kit'