swift-tools-version in Package.swift

[DerekDally]

When trying to import using SPM, I get the following in Xcode 11.3.1:
The package dependency graph can not be resolved; unable find any available tag for the following requirements:
https://github.com/a2/MessagePack.swift — 3.0.1..<4.0.0

Using 11.4 beta 2, I get a different (likely more accurate) error:
because MessagePack.swift >=2.0.0 contains incompatible tools version and root depends on MessagePack.swift 3.0.1..<4.0.0, version solving failed.

Is this as simple as adding "// swift-tools-version:5.0" (or similar) to the Package.swift? If so, I'm happy to submit a PR. Otherwise if this relates to #70 or I'm just not doing something correctly, let me know?

Thanks