-Added note about the /designs subdir in libraries/.

tools/hdltool_readme.txt

@@ -565,6 +565,8 @@ c) libraries/
    . Library of reusable firmware blocks, categorized by function and in which generic functionality is separeted 
      from technology. Within technology another seperation exists between generic technology and hardware-specific
      IP.
+   . [library_catagory]/[library]/designs/
+     . Contains reference designs to synthesize the library block for specific boards.
 
 d) software/
    . Intended for software such as control/monitoring of boards and programs to capture and process board output,