Add migration guide
Add migration guide

Co-authored-by: Danny <8364b57d74c8e4d5e2c8f305ec1107fe28d99398@users.noreply.github.com>
Co-authored-by: Rapptz <5f70d418b94308014c91458e9d9a5b6b5335e238@gmail.com>
rapptz/discord.py,Rapptz/discord.py,Harmon758/discord.py,Harmon758/discord.py