Clarify PIM documentation/README.md regarding Flyway DDL auto-creation

To avoid any misunderstanding, PIM doc should be updated in order to clarify the auto-creation feature provided when enabling Flyway.

PIM only provides auto schema creation for the in-memory database H2 out of the box, by having the "quarkus.flyway.migrate-at-start" enabled.

Due to some Flyways constraints and limitations, we are only using auto schema creation as enabled for H2. For the rest of the databases, customers should have the PIM database schema created beforehand, and then use PIM against that by re-augmenting and disabling the "quarkus.flyway.migrate-at-start" flag at runtime.