Add installation instructions to the generated README

[JohnSundell]

As suggested by @krzysztofzablocki on Twitter, it would be great if the README file that SwiftPlate generates would contain instructions on how to install the generated framework using CocoaPods, Carthage and the Swift Package Manager.

[agisilaos]

Hey @JohnSundell I would love to start working on that Issue! Can you give me some more info about it?

[JohnSundell]

Happy to hear that @agisilaos 🎉

OK, so what I would suggest is to look at for example Wrap (https://github.com/johnsundell/wrap), and make SwiftPlate generate a similar Installation section in the README file it generates. Basically modify the template here: https://github.com/JohnSundell/SwiftPlate/blob/master/Template/README.md.

[agisilaos]

@JohnSundell Yup, got it! Makes sense! ☺️

[agisilaos]

Just made this pull request #49 for this specific issue. Let me know what you think @JohnSundell :)