docs: add info about ios push registry

[mhuynh5757]

Submission Checklist

• Updated the CHANGELOG.md to reflect any feature, bug fixes, or known issues made in the source code
• Tested code changes and observed expected behavior in the example app
• Performed a visual inspection of the Files changed tab prior to submitting the pull request for review to ensure proper usage of the style guide

All third-party contributors acknowledge that any contributions they provide will be made under the same open-source license that the open-source project is provided under.

• I acknowledge that all my contributions will be made under the project's license.

Description

This PR adds some docs regarding incoming calls and iOS push registry set up.

Breakdown

• Add docs.

Validation

• N/A

Additional Notes

N/A

[bobiechen-twilio]

I am still afraid in some cases if the JS runtime is not initialized in time, especially in the cold start situation. The guaranteed way is still doing the PushKit initialization in the iOS AppDelegate (or whichever file that implements the app lifecycle callbacks).
We need to make sure the suggestion works flawlessly otherwise we might need to instruct them how to modify the app-delegate.