Mention using dovecot 2.2+ in Migration section of README

[taoeffect]

To speed up the migration, and to fix migration errors, users should install dovecot from Debian testing or unstable (or whatever's necessary to get version 2.2+).

Speeding up also involves removing the {# fetch-headers #} jinja comment in migration/tasks/main.yml.

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[taoeffect]

Might not be so important given that we'll be using Larch by default (#36).