docs: Add howto to override default http client. (#412)

* Add howto to override default http client.

* updates from PR feedback

* Add badge showing latest version of dependency

* remove redundant word

Author: kggilmer

docs/howto/README.md

@@ -5,4 +5,5 @@ This documentation supplements the official [AWS SDK for Kotlin Developer Guide]
 
 ## Configuration
 
-* [Endpoints](./configuring/endpoints.md) - How to configure a custom endpoint resolver
+* [Endpoints](./configuring/endpoints.md) - How to configure a custom endpoint resolver
+* [Http Clients](./configuring/http-clients.md) - How to specify an alternative HTTP client

docs/howto/configuring/http-clients.md

@@ -0,0 +1,33 @@
+# Configuring HTTP Clients
+
+By default the AWS SDK for Kotlin uses an HTTP client known as the AWS Common Runtime (CRT) HTTP client. This 
+client was written by AWS to ensure the best experience with AWS services. However customers may choose to override 
+the default HTTP client by specifying an existing [HttpClientEngine](https://github.com/awslabs/smithy-kotlin/blob/main/runtime/protocol/http/common/src/aws/smithy/kotlin/runtime/http/engine/HttpClientEngine.kt) 
+implementation or implementing their own, and referencing that implementation in the service client configuration at 
+the time of client construction.
+
+The SDK provides an additional client, `KtorEngine` via the `aws.smithy.kotlin:http-client-engine-ktor` dependency.
+
+[![Maven][maven-badge]][maven-url]
+
+[maven-badge]: https://img.shields.io/maven-central/v/aws.smithy.kotlin/http-client-engine-ktor.svg?label=Maven
+[maven-url]: https://search.maven.org/search?q=g:aws.smithy.kotlin+a:http-client-engine-ktor
+
+## Example
+
+The following code snippet demonstrates constructing an S3 client using the [Ktor OkHttp HTTP client](https://ktor.io/docs/http-client-engines.html#okhttp):
+
+`build.gradle.kts`:
+```kotlin
+dependencies {
+    implementation("aws.smithy.kotlin:http-client-engine-ktor:<version>")
+}
+```
+
+Application code:
+```kotlin
+val sharedConfig = AwsClientConfig.fromEnvironment()
+val client = S3Client(sharedConfig) {
+    httpClientEngine = KtorEngine()
+}
+```