Add a Package.swift manifest

[ma11hew28]

I'd like to add a package dependency to my app in Xcode and enter this repository's URL.

Of course, when I try that now, I get the error:

https://github.com/signalapp/libsignal-protocol-c.git has no Package.swift manifest for version 2.3.3

If this repo had a Package.swift file, then integrating it into my iPhone app would be much easier.

Subtasks

• Remove the CMakeModules/iOS.toolchain.cmake file
• Update the README
  • Change "beyond CMake" to "beyond CMake or Swift Package Manager"
  • Remove the "iOS" bullet point

Fixing this issue would obsolete #16 & #34.

Related Issues

• https://forums.swift.org/t/swiftpm-is-it-possible-to-specify-openssl-as-a-dependency/26568
• https://bugs.swift.org/browse/SR-12687
• https://stackoverflow.com/q/61444720/242933