add help for --with-hcoll

Added by Josh, reviewed by Mike
cmr=v1.7.5:reviewer=ompi-rm1.7

This commit was SVN r30637.

README

@@ -841,6 +841,16 @@ NETWORKING SUPPORT / OPTIONS
 
   FCA is the support library for Mellanox QDR switches and HCAs.
 
+--with-hcoll=<directory>
+  Specify the directory where the Mellanox hcoll library and header
+  files are located.  This option is generally only necessary if the
+  hcoll headers and libraries are not in default compiler/linker
+  search paths.
+
+  hcoll is the support library for MPI collective operation offload on
+  Mellanox ConnectX-3 HCAs (and later).
+
+
 --with-knem=<directory>
   Specify the directory where the knem libraries and header files are
   located.  This option is generally only necessary if the knem headers