Update README with new Xcode information (10.2 beta)

Author: shahmishal

README.md

@@ -72,7 +72,7 @@ supported host development operating systems.
 
 #### macOS
 
-To build for macOS, you need [Xcode 10.0](https://developer.apple.com/xcode/downloads/).
+To build for macOS, you need [Xcode 10.2 beta](https://developer.apple.com/xcode/downloads/).
 The required version of Xcode changes frequently, and is often a beta release.
 Check this document or the host information on <https://ci.swift.org> for the
 current required version.