Specify compatible Swift versions in readme.

We're dropping Swift 3.0 compatibility because Swift 3.0 doesn't understand the `swiftLanguageVersions` key in the Package.swift manifest. The code should still be compatible with Swift 3.0 at this moment. I don't think this is a big deal since moving from Swift 3.0 to Swift 3.1 should be straightforward.

Author: ole

README.md

@@ -1,6 +1,6 @@
 # SortedArray
 
-A sorted array type written in Swift 3.0.
+A sorted array type for Swift 3.1+ and 4.0+.
 
 Provides the `SortedArray` type, an array that keeps its elements sorted according to a given sort predicate.
 
@@ -14,6 +14,8 @@ For more info, see the [GitHub repo](https://github.com/ole/SortedArray) and my
 
 ## Supported Platforms
 
+Supports Swift versions 3.1, 3.2, and 4.0. Not compatible with Swift 3.0. The last version compatible with Swift 3.0 is v0.4.
+
 Since the code has no dependencies other than the Swift standard library (it doesn't even use Foundation), it should work on all platforms where Swift is available.
 
 I tested it on macOS, iOS, tvOS, and Linux.