message: Add instructions on running benchmarks to integration-tests README
diff: # Handwritten Integration Test Root
  -This folder contains hand-written integration tests that are specific to individual services. In order for your test to be merged into the final artifact:
  +This folder contains hand-written integration tests that are specific to
  +individual services. In order for your test to be merged into the final artifact:
  - The crate name must match the generated crate name, eg. `kms`, `dynamodb`
  - Your test must be placed into the `tests` folder. **Everything else in your test crate is ignored.**
  -The contents of the `test` folder will be combined with codegenerated integration tests & inserted into the `tests` folder of the final generated service crate.
  +The contents of the `test` folder will be combined with code-generated integration
  +tests & inserted into the `tests` folder of the final generated service crate.
  +
  +## Benchmarks
  +
  +Some integration test roots have a `benches/` directory. In these, `cargo bench` can be
  +invoked to run the benchmarks against the current version of smithy-rs. To compare
  +across smithy-rs versions, you can use git to checkout the version to compare against,
  +run the benchmark, and then checkout the other version and run it again:
  +
  +```bash
  +# For example, this was the very first commit that had a benchmark
  +git checkout 1fd6e978ae43fb8139cc091997f0ab76ae9fdafa
  +
  +# Re-generate the SDK before benchmarking to make sure you have the correct code
  +./gradlew :aws:sdk:assemble
  +
  +# The DynamoDB integration tests have benchmarks. Let's run them
  +cd aws/sdk/integration-tests/dynamodb
  +cargo bench
  +
  +# Record the results...
  +
  +# Now, run against the latest
  +git checkout main
  +
  +# Re-generate the SDK before benchmarking to make sure you have the correct code
  +cd ../../../..
  +./gradlew :aws:sdk:assemble
  +
  +# Go run the same benchmarks with the latest version
  +cd aws/sdk/integration-tests/dynamodb
  +cargo bench
  +
  +# Compare!
  +```
  