update README.md

Author: adrpo

README.md

@@ -9,7 +9,7 @@ To get everything running, you will need a few extras:
 
 - C++11 compiler (if you want a GUI)
 - autoconf, automake, libtool, g++, gfortran (pretty standard compilers)
-- boost (optional; used with configure --with-cppruntime)
+- boost (used by omsimulator and cppruntime, configure --with-cppruntime)
 - [clang](http://clang.llvm.org/), clang++ (optional, but *highly recommended*)
 - [cmake](http://www.cmake.org)
 - hwloc (optional; queries the number of hardware CPU cores instead of logical CPU cores)