Add comment to explain why we dismiss he PSPDFViewController

[radazzouz]

Details

This is a followup PR for #280. See #280 (comment)

Acceptance Criteria

• Add comment.
• When approved, right before merging, rebase with master and increment the package version in package.json, package-lock.json, and samples/Catalog/package.json (see example commit: 1bf805f).
• Create a new release (and tag) with the new package version (see https://github.com/PSPDFKit/react-native/releases).