Documentation for support of version ranges

[joelanford]

Let's document our newly added support for version ranges being specified in the Operator API's spec.version field.

[tmshort]

Place into docs directory of the operator-controller repo, perhaps a reference or staging folder within that.

[joelanford]

The docs here may have some overlap with the docs described in #470. Linking together for visibility.

[michaelryanpeter]

@tmshort I am happy to collaborate on the docs.