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

diff --git a/bench/README.md b/bench/README.md
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+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.