Properly document the differences and incompatibilities between Fabric8 and Kubernetes Client

[oneiros-de]

Describe the bug
The documentation for Spring Cloud Kubernetes mentions both the Fabcri8 and the Kubernetes Client, but doesn't explain that one can only use one of these, not both. This should be explained in the docs.

Sample
The section on leader election instructs one to add the org.springframework.cloud:spring-cloud-kubernetes-fabric8-leader dependency, but this is incompatible with org.springframework.cloud:spring-cloud-starter-kubernetes-client-config. See this question on Stackoverflow.