Add warning about spaces in the Xcode path.

Author: David Ungar

README.md

@@ -102,7 +102,7 @@ Please make sure you use Python 2.x. Python 3.x is not supported currently.
 To build for macOS, you need [Xcode 12 beta 3](https://developer.apple.com/xcode/resources/).
 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.
+current required version. The path name for the Xcode app must not include any spaces.
 
 You will also need [CMake](https://cmake.org) and [Ninja](https://ninja-build.org),
 which can be installed via a package manager: