Add shared-plugin to the SDK Packages tables docs

`@openshift-console/plugin-shared` (NPM) is a package that will contain shared components that can be upversioned separately by the Plugins so they can keep core compatibility low but upversion and support more shared components as we need them.

This isn't documented today. We need to do that.

Acceptance Criteria

• Add a note in the "SDK packages" section of the README about the existence of this package and it's purpose
  • The purpose of being a static utility delivery library intended not to be tied to OpenShift Console versions and compatible with multiple version of OpenShift Console