Documentation and examples are needed for `type: Selector`

[bobh66]

What happened?

There is no information available in the documentation (README) or examples for type: Selector which is a much more powerful method of selecting EnvironmentConfigs then the reference by name.

How can we reproduce it?

Try to find info on using selectors in the repo - it only exists in the fn_test.go file

[ajski1701]

@bobh66 Should be good to go with #85 being merged in.