Add instructions on running benchmarks to integration-tests README (#522)

# 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!

```