docs: add Lean Core badge

[Naturalclar]

Summary

This PR is part of react-native-community/.github#45

Added LeanCore Extracted Badge in README

Also, modified style of shield badge to be consistent

Test Plan

What's required for testing (prerequisites)?

What are the steps to reproduce (after prerequisites)?

Compatibility

OS	Implemented
iOS	✅❌
Android	✅❌

Checklist

• [ ] I have tested this on a device and a simulator
• [x] I added the documentation in README.md
• [ ] I mentioned this change in CHANGELOG.md
• [ ] I updated the typed files (TS and Flow)
• [ ] I added a sample use of the API in the example project (example/App.js)