Update supported version in README (#1517)
Update supported version in README (#1517)

* Update supported version in README

Motivation:

NIO can run on macOS 10.9+/iOS 7+, but the README states 10.12+.

Modifications:

Update readme.

Result:

More accurate docs.
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio