push action button readme update

[amber-klaviyo]

Description

Update the readme to cover push action buttons- link to native docs for iOS and Android.

Due Diligence

• I have tested this on a simulator/emulator or a physical device, on iOS and Android (if applicable).
• I have added sufficient unit/integration tests of my changes.
• I have adjusted or added new test cases to team test docs, if applicable.
• I am confident these changes are implemented with feature parity across iOS and Android (if applicable).

Release/Versioning Considerations

• Patch Contains internal changes or backwards-compatible bug fixes.
• Minor Contains changes to the public API.
• Major Contains breaking changes.
• Contains readme or migration guide changes.
  • If so, please merge to a feature branch so documentation updates only go live upon official release.
• This is planned work for an upcoming release.
  • If no, author or reviewer should account for this in a release plan, or describe why not below.

Changelog / Code Overview

Test Plan

Related Issues/Tickets

[ajaysubra]

LGTM

[evan-masseau]

Retargeted to rel/2.4.0