bench: add a redirect

BurntSushi · BurntSushi · commit 25ad29f56e10 · 2023-10-13T09:51:09.000-04:00
This directory was linked in a fair number of places, so we re-introduce
it with a README pointing folks toward rebar.
diff --git a/bench/README.md b/bench/README.md
@@ -0,0 +1,2 @@
+Benchmarks for this crate have been moved into the rebar project:
+https://github.com/BurntSushi/rebar

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,2 @@`
	`1`	`+Benchmarks for this crate have been moved into the rebar project:`
	`2`	`+https://github.com/BurntSushi/rebar`