docs(aws-smithy-mocks): Document that client packages should enable `test-util` feature, provide sample `Cargo.toml`

[jlizen]

It's not obvious from the aws-smithy-docs (module-level at least), that you need a test-util feature enabled on any client packages. Without it, you get something like:

no method named with_test_defaults found for struct aws_sdk_s3::config::Builder in the current scope
method not found in Builder

The correct usage would look something like:

[dependencies]
aws-sdk-s3 = "1"

[test-dependencies]
aws-smithy-mocks = "0.1"
aws-sdk-s3 = { version = "1", features = ["test-util"] }

It would be great to document this more prominently, with a sample Cargo.toml. Might also be nice to include the error message as a searchable breadcrumb.