Add a notice/doc to warn about not puting a config.json at the root folder

[hervelemeur]

Summary

It took me a while to debug it but with the last jx versions you shouldn't have a config.json file at the root folder of your app, because if there is one, it will be used by kaniko to configure the docker registry, and depending on its content it will almost certainly cause a build error at the container-build step.

What put me on the right track after having commented all my overriden steps in my jenkins-x.yaml (with one of them copying a config.example.json to config.json): GoogleContainerTools/kaniko#887

A month ago it didn't cause any problem for this project, and my other projects don't have any problem at all.

Steps to reproduce the behavior

Have a config.json in the root folder of your app with anything but a kaniko config in it, like:

{
    "my-own-parameter": "true"
}

Expected behavior

A correct build.

Actual behavior

Build error at the step "container-build" in the "build" stage of the "release" pipeline:

Showing logs for build my-org/my-app/master #1 stage from-build-pack and container step-build-container-build
error checking push permissions -- make sure you entered the correct tag name, and that you are authenticated correctly, and try again: checking push permission for "my-registry.azurecr.io/my-org/my-app:0.0.1": creating push check transport for my-registry.azurecr.io failed: unsupported status code 401

Jx version

The output of jx version is:

NAME               VERSION
jx                 2.0.1284
Kubernetes cluster v1.15.7
kubectl            v1.17.3
helm client        2.16.3
git                2.21.1 (Apple Git-122.3)
Operating System   Mac OS X 10.15.3 build 19D76

Jenkins type

• Serverless Jenkins X Pipelines (Tekton + Prow)
• Classic Jenkins

Kubernetes cluster

AKS