Update README to include UICollectionView introspection support

[ryohey]

This PR updates the README to include the newly added introspectCollectionView() and introspectCollectionViewCell() functions from PR #169. It also clarifies that these new functions are available for Lists on iOS 16 and above, while the introspectTableView() and introspectTableViewCell() functions are available for iOS 15 and below.

- Added `introspectCollectionView()` and `introspectCollectionViewCell()` functions to the README
- Clarified that the new functions are available for Lists on iOS 16 and above
- Specified that `introspectTableView()` and `introspectTableViewCell()` are available for iOS 15 and below

Please let me know if any changes or additional information are required. Thank you!

[davdroman]

Thanks!