Add a summary of package usage in client README

[trivikr]

Is your feature request related to a problem? Please describe.
The client README doesn't contain client specific information. For example, the README from @aws-sdk/client-s3 just asks user to refer to generic root README.

Screenshot

The README is used as a landing page at three places:

• GitHub: https://github.com/aws/aws-sdk-js-v3/tree/master/clients/client-s3
• API Reference: https://docs.aws.amazon.com/AWSJavaScriptSDK/v3/latest/clients/client-s3/index.html
• Npm website: https://www.npmjs.com/package/@aws-sdk/client-s3

Describe the solution you'd like
Populate README providing detailed documentation like it was done for preview clients: https://www.npmjs.com/package/@aws-sdk/client-s3-node

Describe alternatives you've considered

• Using generic README on repository root to understand how specific client works.
• Dig into the API Reference: https://docs.aws.amazon.com/AWSJavaScriptSDK/v3/latest/clients/client-s3/classes/s3.html

Additional context
Refs: #1877 (comment)

[trivikr]

README for preview clients was populated in

aws-sdk-js-v3/packages/service-types-generator/src/clientReadme.ts

Line 18 in 3c06325

export function readme({ model, name: packageName, runtime }: ReadmeInterface) {

README for smithy clients is populated in

aws-sdk-js-v3/codegen/smithy-aws-typescript-codegen/src/main/java/software/amazon/smithy/aws/typescript/codegen/AwsPackageFixturesGeneratorIntegration.java

Lines 51 to 55 in 96f1087

	writerFactory.accept("README.md", writer -> {
	String resource = IoUtils.readUtf8Resource(getClass(), "README.md.template");
	resource = resource.replaceAll(Pattern.quote("${packageName}"), settings.getPackageName());
	writer.write(resource);
	});

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