Added Swift Package Manager Support

[Ganesh623]

Hi,
Added Swift Package Manager Support. Check, If everything fine. Merge it. It's very useful for latest projects.

Thanks

[sauvikdolui]

@Ganesh623 thanks for expressing interest in this tiny repository, did not wish that even it can cross a star count of 125. Thanks for this pull request, and I sincerely apologize for being so late to respond because of busy schedule. Yes I agree, support of SPM will help those we are not favor of installing dependencies using Cocoa Pods. There are a few issues.

Let's come to the PR

1. I can see the changes but, I do not see them well organized also the source file (./Sources/SDStateTableView/SDStateTableView.swift) contains the template file contents.

2. Can you try to organize the source file in such a way so that the source files are not duplicated and both cocoapod and spm can use them?
3. As you can see there is an example project which demonstrates how to use this library using cocoapods, i guess that will also be helpful for users to quickly install and start using this library in their own projects.
4. And finally not the least, please also update ReadMe file with a new section under installation to instruct users how to use it.

The above four are crucial for easy maintenance and ease of end-users. I hope you will agree to these points and try to update your PR accordingly. This forum answer might be helpful for you https://forums.swift.org/t/how-to-create-a-c-library-that-supports-both-cocoa-pods-and-spm/24907.

Thanks.