Not clear what the upgrade plan module does

[vburckhardt]

terraform-ibm-container-registry/modules/plan/README.md

Line 3 in 246bd19

You can use this submodule to upgrade the IBM [Container Registry](https://cloud.ibm.com/docs/Registry?topic=Registry-getting-started) plan.

Link in particular is pointing to getting started page. Would help to clearly indicate what this is for.

[shemau]

The best information I could find to assist is at https://cloud.ibm.com/docs/Registry?topic=Registry-registry_overview#registry_plans. This will be included in the next to #275.