Add README.md for arith-stats.

Signed-off-by: Tim Callahan <[email protected]>

Author: tcal-x

arith-stats-plugin/README.md

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+
+# arith-stats
+
+Counts the number of arithmetic cells in each module, also providing a histogram by operand bitwidths.
+
+
+* Cell types counted:
+    `$add $sub $neg $mul $div $lt $le $gt $ge $alu $fa $macc`
+
+
+* Usually you want to run at least `hierarchy -top <top>`, `proc`, `flatten`, and `clean` before calling `arith_stats`.
+
+* If the design is not flattened, the counts per module refer only to the arithmetic cells directly in that module, not those contained in instantiations of other modules.
+
+* If you run early in the flow, you will see `$add/$sub/$mul/$div` and comparisons.
+
+* If you run after `alumacc`, the count will contain `$alu` and `$macc` cells.
+
+* If you run after `maccmap`, you will see `$fa` and `$alu` counts.
+
+* WARNING: the bitwidth for $macc cells is not currently correct since it doesn't parse the config of multiple inputs.
+
+
+This plugin uses C++17.
+
+Sample script:
+```
+read_verilog test.v
+hierarchy -top top
+proc
+flatten
+
+plugin -i arith-stats
+arith-stats
+