Created interfaces with documentation

[00benallen]

• Scaffolded the swift project using SPM and Xcode
• Created the 4 Reactive Streams interfaces in Swift, using protocols
• Added documentation in-code and updated the Readme

[viktorklang]

@00benallen Hi! Thanks for taking the time, and showing interest in porting Reactive Streams to Swift.
I had a look at this PR, and I'm definitely not qualified to make any comments as to what files need to go into a Swift-based GH project. Some of the other @reactive-streams/contributors might have more thoughts/feedback here :)

In general, we've never had author-tags in the source (to encourage collective ownership and responsibility), and I guess you're going to want to either refer to, or port the spec to this repo too.

Then of course, the large body of work will be to port the TCK (In my mind basically the most important part besides the spec and the interfaces)!