Migration guide

[axelson]

Thanks for maintaining kronky/this! Is there a migration guide for migrating from Kronky? From the sounds of it not much is needed, just changing {:kronky, ">= 0.0.0"} in your deps to :absinthe_error_payload and Kronky to AbsintheErrorPayload but it would be nice to have that explicitly called out in the Readme or supplementary doc.