Add another set of convenient commands to readme

[ivanfilippov]

Add a couple of commands to make adding custom resources easier.

[Paxa]

Should we make option to backup all crd's?

[ivanfilippov]

I think that's a good idea. I see it as being a new env var config, example below:
ALL_GLOBAL_RESOURCES = true|false
ALL_RESOURCES = true|false

I think it's still important to allow the use of the SKIP_* configs with the ALL_* configs to allow users to exclude certain resources.

I have not touched Ruby before, let me know if this is something you'll work on. If not, I can leave this PR open (or convert it into an issue) and look into picking up Ruby.

[Paxa]

That would be nice, can implement via

kubectl api-resources --verbs=list --namespaced=false -o name # global list
kubectl api-resources --verbs=list --namespaced=true -o name # namespaced

Will try to make it later