README updates

README.md

@@ -329,6 +329,20 @@ Verilog Attributes and non-standard features
   that represent module parameters or localparams (when the HDL front-end
   is run in -pwires mode).
 
+- The ``clkbuf_inhibit`` attribute can be set on a wire to prevent
+  automatic clock buffer insertion by ``clkbufmap``.
+
+- The ``clkbuf_sink`` attribute can be set on an input port of a blackbox
+  module to request clock buffer insertion by the ``clkbufmap`` pass.
+
+- The ``clkbuf_driver`` attribute can be set on an output port of a blackbox
+  module to mark it as a clock buffer output, and thus prevent ``clkbufmap``
+  from inserting another clock buffer on a net driven by such output.
+
+- The ``iopad_external_pin`` attribute on a blacbox module's port marks
+  it as the external-facing pin of an I/O pad, and prevents ``iopadmap``
+  from inserting another pad cell on it.
+
 - In addition to the ``(* ... *)`` attribute syntax, Yosys supports
   the non-standard ``{* ... *}`` attribute syntax to set default attributes
   for everything that comes after the ``{* ... *}`` statement. (Reset