pls ad to readme: if using postgres (Heroku), use :string not :enum in migrations

[jpwynn]

I found a note to this in an old issue, but it really ought to be in the readme because ...

by the time you push to heroku,
hit the error,
FIND the error,
then fix the error several commits back in your repo's to fix the migration (have to go back in history to fix it, can't add another migration later to fix it because the migration with :enum stops further migrations) is a real pain.

For that matter, is there any reason to recommend using :enum instead of :string in migrations in the first place?