Add CHANGELOG entry, add abc9_{flop,keep} attr to README.md

CHANGELOG

@@ -57,6 +57,7 @@ Yosys 0.9 .. Yosys 0.9-dev
       always_latch and always_ff)
     - Added "xilinx_dffopt" pass
     - Added "scratchpad" pass
+    - Added "synth_xilinx -dff"
 
 Yosys 0.8 .. Yosys 0.9
 ----------------------

README.md

@@ -378,6 +378,12 @@ Verilog Attributes and non-standard features
   for example, to specify the clk-to-Q delay of a flip-flop for consideration
   during techmapping.
 
+- The module attribute ``abc9_flop`` is a boolean marking the module as a
+  whitebox that describes the synchronous behaviour of a flip-flop.
+
+- The cell attribute ``abc9_keep`` is a boolean indicating that this black/
+  white box should be preserved through `abc9` mapping.
+
 - The frontend sets attributes ``always_comb``, ``always_latch`` and
   ``always_ff`` on processes derived from SystemVerilog style always blocks
   according to the type of the always. These are checked for correctness in