STYLE: Added reference to documentation in CMakeLists.txt.

README.txt

 KWSys provides a platform-independent API to many common system
 features that are implemented differently on every platform.  This
-library is intended to be shared among many projects.
+library is intended to be shared among many projects, so it has a
+configurable namespace.  Each project should configure KWSys to use a
+namespace unique to itself.  See comments in CMakeLists.txt for
+details.
 
 You are probably reading this file in the source tree of a surrounding
 project.  In that case, see "../README.kwsys" for details of using