Add README for S3TransferManager

Author: zoewangg

services-custom/s3-transfer-manager/README.md

@@ -0,0 +1,64 @@
+## Overview
+
+This project provides a much improved experience for S3 customers needing to easily perform uploads and downloads of
+objects to and from S3 by providing the S3 S3TransferManager, a high level
+library built on the [AWS Common Runtime S3 Client](https://github.com/awslabs/aws-crt-java).
+
+## Getting Started
+
+### Add a dependency for the transfer manager 
+
+First, you need to include the dependency in your project.
+
+```xml
+<dependency>
+  <groupId>software.amazon.awssdk</groupId>
+  <artifactId>s3-transfer-manager</artifactId>
+  <version>${awsjavasdk.version}-PREVIEW</version>
+</dependency>
+```
+
+Note that you need to replace `${awsjavasdk.version}` with the latest
+SDK version
+
+### Instantiate the transfer manager
+You can instantiate the transfer manager easily using the default settings
+
+```java
+
+S3TranfserManager tranferManager = S3TransferManager.create();
+    
+```
+
+If you wish to configure settings, we recommend using the builder instead:
+```java
+S3TransferManager transferManager = 
+    S3TransferManager.builder()
+                     .s3ClientConfiguration(b -> b.credentialsProvider(credentialProvider)
+                                                  .region(Region.US_WEST_2)
+                                                  .targetThroughputInGbps(20.0)
+                                                  .minimumPartSizeInBytes(10 * MB))
+                     .build();
+```
+
+### Download an S3 object to a file
+To download an object, you just need to provide the destion file path and the `GetObjectRequest` that should be used for the download.
+
+```java
+Download download = transferManager.download(b -> b.destination(path)
+                                   .getObjectRequest(r -> r.bucket("bucket")
+                                                           .key("key")));
+download.completionFuture().join();
+```
+
+### Upload a file to S3
+To upload a file to S3, you just need to provide the source file path and the `PutObjectRequest` that should be used for the upload.
+
+```java
+Upload upload = transferManager.upload(b -> b.source(path)
+                                             .putObjectRequest(r -> r.bucket("bucket")
+                                                                     .key("key")));
+
+upload.completionFuture().join();
+
+```