Mention hwloc in the requirements section of the README

README

@@ -49,6 +49,11 @@ advantage of their specificities in a portable fashion.
    * autoconf (version >= 2.60)
    * automake
 
+ * Remark: It is strongly recommanded that you also install the hwloc library
+   before installing StarPU. This permits StarPU to actually map the processing
+   units according to the machine topology. For more details on hwloc, see
+   http://www.open-mpi.org/projects/hwloc/ .
+
 ++=====================++
 || III. Getting StarPU ||
 ++=====================++