Readme, add hint to XCode <= v9.2

Author: tcm-marcel

README.md

@@ -33,7 +33,7 @@ Check out the [installation instructions](https://github.com/cmu-db/peloton/wiki
 Peloton has been tested to work on the following platforms:
 
 * Ubuntu Linux 14.04+ (64-bit) [gcc4.8, gcc5]
-* Mac OS X 10.9+ (64-bit) [XCode v8 Only]
+* Mac OS X 10.9+ (64-bit) [XCode v8, XCode <= v9.2]
 
 Support for Fedora was dropped because of missing official support for Tensorflow. Please note that it will not compile on 32-bit systems.