Added sentence about nodes in libraries.

tools/hdltool_readme.txt

@@ -571,8 +571,9 @@ b) boards/
 
 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.
+     from technology. These libraries also contain the reusbale node blocks that are used in designs. For instance
+     the bf_lib also contains the node_unb1_fn_bf node that is used in the reference design unb1_fn_bf. 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.