Docs: Section on Upgrading to Packaging CRs from Existing App CRs

[danielhelfand]

Users of kapp-controller who have been using App CRs for existing workloads may eventually want to change to using packaging CRs with kapp-controller. There should be kapp-controller docs created that point out key factors to accomplish this update. The use case of these docs is captured in #184 and this issue may depend on the outcome of this comment in #184.

Some things to consider when writing these docs are:

• How to replicate the App spec when creating a Package CR in order to preserve the existing App CR?
• How InstalledPackages know what App to use if the App CR already exists (i.e. the InstalledPackage should have the same name as the App CR)
• Where to create the InstalledPackage (i.e. in the App CR namespace)