README: mention thread-safety option

Author: bonachea

README.md

@@ -115,7 +115,7 @@ to ensure that a matching set of LLVM compilers are used.
 
 The `install.sh` recognizes a number of command-line options and environment variables to
 customize behavior for your system. See the output of `./install.sh --help` for full documentation,
-including options for how to build for a distributed-memory platform.
+including options for how to build for a distributed-memory platform or with thread-safety.
 
 
 Example Usage