readme: mention limitation of backup with key renewal

[willbush]

Description of the change

Add a note about the limitation of backing up the public / private key with key renewal.

Benefits

Clarity about the limitation of backing up without understanding how key renewal works.

Possible drawbacks

Conciseness of readme

Applicable issues

none

Additional information

Tested in minikube.

• create secret
• backup sealedsecrets.bitnami.com/sealed-secrets-key to main.key
• force renewal
• create secret2
• backup sealedsecrets.bitnami.com/sealed-secrets-key to main2.key
• delete cluster and recreate
• restore main.key and secert2 cannot decrypt.
• restore main2.key and both secrets work

[alemorcuq]

Thanks!