Add Carthage to .gitignore

[giginet]

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've been mindful about doing atomic commits, adding documentation to my changes, not refactoring too much.
• I've a descriptive title and added any useful information for the reviewer. Where appropriate, I've attached a screenshot and/or screencast (gif preferrably).
• I've written tests to cover the new code and functionality included in this PR.
• I've read, agree to, and signed the Contributor License Agreement (CLA).

PR Summary

When I installed this library by Carthage with --use-submodules option, this repository is marked as dirty because of Carthage/ is not ignored.

[dzenbot]

This is great @giginet, although we need you to complete the Contributor License Agreement (CLA) before I can merge this PR.

[giginet]

Thank you!
I filled CLA. Please check it out.

[dzenbot]

Thank you 👍