Update Registry Library docs

[kim-tsao]

Please specify the area for this PR

/area registry

What does does this PR do / why we need it:
Updates the API docs and README to reflect changes in the registry library API over the past several months

Which issue(s) this PR fixes:

Fixes #?
devfile/api#684

PR acceptance criteria:

• Test (WIP)

• [x ] Documentation (WIP)

How to test changes / Special notes to the reviewer:

[openshift-ci]

@kim-tsao: The label(s) area/registry cannot be applied, because the repository doesn't have them.

In response to this:

Please specify the area for this PR

/area registry

What does does this PR do / why we need it:
Updates the API docs and README to reflect changes in the registry library API over the past several months

Which issue(s) this PR fixes:

Fixes #?
devfile/api#684

PR acceptance criteria:

• Test (WIP)

• [x ] Documentation (WIP)

How to test changes / Special notes to the reviewer:

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: johnmcollier, kim-tsao

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