Add missing `kamal init` step to README setup instructions

[hiendinhngoc]

This PR updates the setup instructions in the README to include the missing kamal init step.

Previously, users were asked to edit .kamal/secrets, but this file isn't created until after running kamal init. By adding this step, the setup instructions are now accurate and prevent confusion for new users.

Changes:

• Added step to run kamal init before editing config files.
• Clarified that this command generates the .kamal directory and required config files.