docs: describe jobs

[eliobischof]

It is currently not very clear how the chart makes sure that Zitadel is correctly migrated to newer versions.
Therefore, we add paragraph to the README.md

Additionally, we replace ZITADEL by Zitadel in the README.md

Definition of Ready

• I am happy with the code
• Short description of the feature/issue is added in the pr description
• PR is linked to the corresponding user story
• Acceptance criteria are met
• All open todos and follow ups are defined in a new ticket and justified
• Deviations from the acceptance criteria and design are agreed with the PO and documented.
• No debug or dead code
• My code has no repetitions
• Documentation/examples are up-to-date
• All non-functional requirements are met
• If possible, the test configuration is adjusted so acceptance tests cover my changes