Document migrations concept

[codeliner]

• Event store migrations can be made with ZfSimpleMigrations
• System db migrations can be made with Doctrine

How can we bring both concepts together? Own migration module?

• Would be more cost but meets the needs best

Or run both migration tools?

• ES and system db are separated so why not use different migration systems as well?
• More complex
• less cost

[codeliner]

For a first simple solution it is enough to only use doctrine migrations and create the process_stream with it, too. Prooph link became only a skeleton so it is up to the developer how he/she will manage migrations if system db and event store don't share the same database. But we should document this!