docs: add info about installing extras

[tarilabs]

Description

Resolves #370

How Has This Been Tested?

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[tarilabs]

cc @isinyaaa wdyt?

[tarilabs]

addresses the comment from #370

[isinyaaa]

Looks great Matteo! I'd just add that remark also on the error string under model_registry.ModelRegistry.register_hf_model e.g.:

https://github.com/kubeflow/model-registry/pull/375/files#diff-ed25c8c4e927ca78d91721d179bfe9c0bac9a3e424f417f21d25d0c9363eedc6

[rareddy]

/lgtm
/approve

[google-oss-prow]

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