Document secret needs

Install instructions are missing any reference to the secret required to run Ackee. The logical place for this would be in the prerequisites section of the readme, which is currently empty. This will help users more quickly realize chart doesn't ship with a database (ref: #1), something that'd be useful to list in the prerequisites section along with the secret.

Example secret:

kind: Secret
apiVersion: v1
metadata:
  name: ackee-password
  namespace: ackee
data:
  ACKEE_PASSWORD: ac2zyNAwDag=
type: Opaque

May be created with kubectl create secret.

[suda]

That's a good call. Would you like to do a PR adding support for a secret?

Once a DB is added and I can test end-to-end locally via K3s, I'd be happy to add a pull for the Secret.