Elaborate / emphasize that Kedge is an "extension" of Kubernetes.

[cdrage]

One of the best features of Kedge is the fact that you are able to add Kubernetes constructs easily to the Kedge file.

For example:

• The entire Kedge file is an extension of PodSpec.
• Most of the Kedge objects that we have are also extendable (for example, service is extended from serviceSpec).

We need to emphasize this in the README as well as the documentation that the Kedge-specific keys are there to help and make your life easier, however, you can still add Kubernetes-specific constructs to your file.

[cdrage]

Fix in #437