Document namespace behavior

[Quentin-M]

With both yaml and jsonnet formats, the metadata: namespace value in the manifests is overwritten with the value from --namespace (or default), after templating. Also, $.variables.namespace is available.

It should be documented as it is not intuitive.

https://github.com/coreos/kpm/blob/b26723c/kpm/kub.py#L132
https://github.com/coreos/kpm/blob/b26723c/kpm/jsonnet/lib/kpm.libjsonnet#L61

[Quentin-M]

This also applies to metadata: name.