Should add --HEAD option when install mozart2 using brew

Otherwise brew would complain that mozart2 is a head-only formula.

Author: suzaku

README.MacOS.md

@@ -17,7 +17,7 @@ If you want to work on Mozart 2, follow instructions in the next section instead
 
 ```shell
 brew tap eregon/mozart2
-brew install mozart2
+brew install --HEAD mozart2
 ```
 
 If something goes wrong, it might be intersting to check the contents of the logs