docs: Added iOS notes to supported platforms.

[breautek]

Platforms affected

Docs

Motivation and Context

Updated iOS supported platform documentation to hopefully clarify some confusion as shown here and here

Description

Add mention that if you're using cordova-ios@5 then you should remain on version 5 of this plugin. If you're using cordova-ios@6 then you should be using version 6 of this plugin, or if you're developing exclusively for iOS, then it's safe to uninstall this plugin altogether.

Please let me know if the wording should be altered.

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary

[timbru31]

LGTM and is understandable 👍