add first benchmark

bench/README.md

@@ -0,0 +1,34 @@
+Benchmarks for tui-textarea using [Criterion.rs][criterion].
+
+To run all benchmarks:
+
+```sh
+cargo bench --benches
+```
+
+To run specific benchmark suite:
+
+```sh
+cargo bench --bench insert
+```
+
+To filter benchmarks:
+
+```sh
+cargo bench append::1_lorem
+```
+
+To compare benchmark results with [critcmp][]:
+
+```sh
+git checkout main
+cargo bench -- --save-baseline base
+
+git checkout your-feature
+cargo bench -- --save-baseline change
+
+critcmp base change
+```
+
+[criterion]: https://github.com/bheisler/criterion.rs
+[critcmp]: https://github.com/BurntSushi/critcmp