Update README.md

[wormeyman]

Add steps for migrations both ways.

First time contributor checklist:

• [ ] I have read the README and Contributor Guidelines
• [ ] I have signed the Contributor Licence Agreement

Contributor checklist:

• [ ] My contribution is not related to translations. Please submit translation changes via our Signal Desktop Transifex project.
• [ ] My commits are in nice logical chunks with good commit messages
• [ ] My changes are rebased on the latest development branch
• [ ] A yarn ready run passes successfully (more about tests here)
• [ ] My changes are ready to be shipped to users

Description