[Migration] add note for migration to v2 of propel migration

[marcj]

v1 dealt with migration in the way that it only stored the latest executed migration timestamp. In v2 we introduced parallel migrations which stores basically every executed migration in our migration table.

If a project now switches from v1 to v2, you have to mark all your migration files as already executed using the new introduced migration option --fake, otherwise the new migration would execute all migrations again (which results in most of the cases in fatal errors, in a corrupt database, or invalid content). We should add a note regarding this issue in our Whats-New page.