Support of Swift Package Manager

[TwoDollarsEsq]

Checklist

• [x] I've tested my changes(demo branch).
• [x] I've read the Contribution Guidelines.
• [x] I've updated the documentation if necessary.

Motivation and Context

This change allows to use this library with Swift Package Manager. Since Xcode 11 using SPM is extremely easy due to its integration into IDE, so it is good thing for every framework that is targeted to platforms that support Swift to be able to be used with SPM.

Description

Adds Package.swift file which enables usage with SPM. Note, that the most correct usage with SPM will be possible only when there is new release that includes Package.swift. Until then, dependency can be added by branch or commit hash which is not allowed in published packages.

[CaliCastle]

Thank you so much for the contribution!