Update README to mention support for Linux.
Update README to mention support for Linux.

watson-developer-cloud/ios-sdk,tad-iizuka/swift-sdk,watson-developer-cloud/ios-sdk,tad-iizuka/swift-sdk,watson-developer-cloud/ios-sdk,tad-iizuka/swift-sdk,tad-iizuka/swift-sdk,watson-developer-cloud/ios-sdk,watson-developer-cloud/ios-sdk,tad-iizuka/swift-sdk