ENH: Added documentation for ITK.

README.itk

+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.
+
+How to use KWSys from ITK:
+
+#include <itkkwsys/SystemTools.hxx>
+
+bool MyIsDirectory(const char* fname)
+{
+  return itkkwsys::SystemTools::FileIsDirectory(fname);
+}
+
+How to modify KWSys:
+
+When adding a method to an existing class, make no mention of ITK.
+KWSys has no knowledge of ITK headers or libraries.  Please also keep
+in mind that KWSys must be able to build on more platforms and
+compilers than ITK.  Refer to existing code for conventions to ease
+this task.
+
+If you want to add a class, please contact the ITK mailing list for
+discussion.  Please do not add a class without permission from
+Kitware.