document migration issue with email field

[guruofgentoo]

We use the EmailType to store emails. Because emails are not case-sensitive, that type converts to lowercase before persisting the value to the database.

When migrating an existing database to keg-auth user model, emails stored there may not be lowercase. That breaks the expectations of EmailType for comparisons, and things like login and the forgot password form will not find the records.

We need to at least document this and related migration issues (such as #160). And, it may be helpful to provide a migration-related CLI task that will resolve these common issues.