Help: Add notes for topic 'link_what_you_use'

Help/release/dev/link_what_you_use.rst

@@ -0,0 +1,7 @@
+link_what_you_use
+-----------------
+
+* A :prop_tgt:`LINK_WHAT_YOU_USE` target property and supporting
+  :variable:`CMAKE_LINK_WHAT_YOU_USE` variable were introduced
+  to detect (on UNIX) shared libraries that are linked but not
+  needed by running ``ldd -r -u``.