rsz: added -verbose argument in README.md

Author: jhkim-pii

src/rsz/README.md

@@ -208,14 +208,15 @@ The `buffer_ports -inputs` command adds a buffer between the input and its
 loads.  The `buffer_ports -outputs` adds a buffer between the port driver
 and the output port. Inserting buffers on input and output ports makes
 the block input capacitances and output drives independent of the block
-internals.
+internals. It uses the buffer library cell defined by `-buffer_cell` if it is given. 
 
 ```tcl
 buffer_ports 
     [-inputs] 
     [-outputs] 
     [-max_utilization util]
     [-buffer_cell buf_cell]
+    [-verbose]
 ```
 
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