Add a README-BLIF.txt file for the blif target.

tgt-blif/README-BLIF.txt

@@ -0,0 +1,44 @@
+
+BLIF TARGET
+-----------
+
+The BLIF code generator supports emitting the design to a blif format
+file as accepted by:
+
+    ABC: A System for Sequential Synthesis and Verification
+    <http://www.eecs.berkeley.edu/~alanmi/abc/>
+
+This package contains tools sometimes used by ASIC designers. This
+blif target emits .blif file that the ABC system can read int via
+the "read_blif" command.
+
+
+USAGE
+-----
+
+This code generator is intended to process structural Verilog source
+code. To convert a design to blif, use this command:
+
+    iverilog -tblif -o<path>.blif  <source files>...
+
+The source files can be Verilog, System Verilog, VHDL, whatever Icarus
+Verilog supports, so long as it elaborates down to the limited subset
+that the code generator supports. In other words, the files must be
+structural.
+
+The root module of the elaborated design becomes the model is
+generated. That module may instantiate sub-modules and so on down the
+design, completing the design. The output model is flattened, so it
+doesn't invoke any subcircuits. Bit vectors are exploded out at the
+model ports and internally. This is necessary since blif in particular
+and ABC in general processes bits, not vectors.
+
+
+LIMITATIONS
+-----------
+
+Currently, only explicit logic gates and continuous assignments are
+supported.
+
+The design must contain only one root module. The name of that root
+module becomes the name of the blif model in the ".model" record.