Add trivial README to bench to describe how to run them.

Author: TheBlueMatt

bench/README.md

@@ -0,0 +1,6 @@
+This crate uses criterion to benchmark various LDK functions.
+
+It can be run as `RUSTFLAGS=--cfg=ldk_bench cargo bench`.
+
+For routing or other HashMap-bottlenecked functions, the `hashbrown` feature
+should also be benchmarked.