ENH: Added XXX_RUNTIME_LIBRARY_DIRS as a suggested variable.

Modules/readme.txt

@@ -15,6 +15,9 @@ XXX_VERSION_YY		Expect Version YY if true. Make sure at most one of these is eve
 XXX_WRAP_YY		If False, do not try to use the relevent CMake wrapping command.
 XXX_YY_FOUND           	If False, optional YY part of XXX sytem is not available.
 XXX_FOUND              	Set to false, or undefined, if we haven't found, or don't want to use XXX.
+XXX_RUNTIME_LIBRARY_DIRS Optionally, the runtime library search path for use when running an executable linked to shared libraries.
+                         The list should be used by user code to create the PATH on windows or LD_LIBRARY_PATH on unix.
+                         This should not be a cache entry.
 
 You do not have to provide all of the above variables. You should provide XXX_FOUND under most circumstances. If XXX is a library, then  XXX_LIBRARIES, should also be defined, and XXX_INCLUDE_DIRS should usually be defined (I guess libm.a might be an exception)