Add some notes about maintaining the hwloc framework.

This commit was SVN r25132.

opal/mca/hwloc/README.txt

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+12 Sep 2011
+
+Notes for hwloc component maintainers:
+
+1. There can only be *1* hwloc version component at a time.
+   Specifically: if there are multiple hwlocXYZ components (i.e.,
+   different versions of hwloc), then they must all be .ompi_ignore'd
+   except for 1.  This is because we currently m4_include all of the
+   underlying hwloc's .m4 files -- if there are multiple hwlocXYZ
+   components, I don't know if m4 will barf at the multiple,
+   conflicting AC_DEFUNs, or whether it'll just do something
+   completely undefined.
+
+1a. As a consequence, if you're adding a new hwloc version component,
+   you'll need to .ompi_ignore all others while you're testing the new
+   one. 
+
+2. If someone wants to fix #1 someday, we might be able to do what we
+   do for libevent: OMPI_CONFIG_SUBDIR (instead of slurping in hwloc's
+   .m4 files).