docs: add docs for benchmark

benchmark/README.md

+# Setup Profile
+
+[Intro to rustc's self profiler](https://blog.rust-lang.org/inside-rust/2020/02/25/intro-rustc-self-profile.html)
+
+```
+$ cargo install --git https://github.com/rust-lang/measureme crox flamegraph summarize
+```
+
+```
+rustup override set nightly
+```
+
+
+## Profile
+
+```bash
+cargo rustc -- -Zself-profile
+```
+
+```
+{crate name}-{rustc process id}.{events,string_data,string_index}.
+```
+
+then
+
+```bash
+summarize summarize regex-17088
+```
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