add warning that configuration with environment variables for cluster wont show in service.json

[petscheit]

As I learned today, when configuring the ipfs-cluster-service with environment variables not all changes will be written to service.json, while others are.
For example, when setting the CLUSTER_SECRET environment variable, the secret will be written to the service.json file. When setting other variables like CLUSTER_REPLICATIONFACTORMIN the changed will take affect, but wont be written to service.json. I think it's pretty confusing and should be mentioned in the docs.

[hsanjuan]

Hi, we are working on a better approach at #663 where behaviour will be what you expect.

[hsanjuan]

Just merged #663 so this should be fixed.