Migration guide for 1.0

[rasmus]

Since 1.0 marks an important milestone for EventFlow, its important that the there's a migration guide in place to help developers upgrade. The expectations are that there will be few braking changes to the EventFlow APIs, but there will be questions for data integrity as well as recommendations on how to actually do the upgrade.

Initial work is done here. https://github.com/eventflow/EventFlow/blob/develop-v1/MIGRATION_GUIDE.md