Add benchmark script mentioned in README.md

bench.sh

@@ -0,0 +1,14 @@
+#!/bin/sh --
+set -euf
+unset IFS RUST_BACKTRACE
+if test $# -ne 1; then
+   printf 'Must have exactly one argument\n' >&2; exit 1
+fi
+case $1 in
+   (*[!a-f0-9]*) printf 'Invalid character in hex argument\n' >&2; exit 1;;
+esac
+if test "$((${#1} & 1))" -ne 0; then
+   printf 'Argument must be of even length\n' >&2; exit 1
+fi
+
+VDF_BENCHMARK_SEED=$1 cargo bench -p vdf --bench classgroup-bench