[Feedback]: Add metadata to pubspec.yaml

[cgutierr-zgz]

What's on your mind?

Add metadata so it's easier to go to the repo/issues from pub.dev

Example:

Code of Conduct

• I agree to follow this project's Code of Conduct

[emawby]

Great suggestion we will look into this

[nan-li]

Hi @cgutierr-zgz,

Thanks for making this issue and corresponding PR. I noticed that these links are already working on https://pub.dev/packages/onesignal_flutter. Are these PR changes still needed?

[cgutierr-zgz]

Sup @nan-li
Back in the day, they were not showing, prolly an update that automatically detects if contributing.md / kind of repo it is has been rolled out and that's why it shows now, so idk prolly not needed :)

[jennantilla]

We'll go ahead and close this issue and corresponding PR #758 since it has been resolved. Thanks!