Add initial design tenets/goals for S3 TransferManager

Author: dagnir

docs/design/services/s3/transfermanager/README.md

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+# Project Tenets (unless you know better ones)
+
+1. Meeting customers in their problem space allows them to deliver value
+   quickly.
+2. Meeting customer expectations drives usability.
+3. Discoverability drives usage.
+
+# Introduction
+
+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 S3 client.
+
+# Project Goals
+
+1. For the use cases it addresses, i.e. the transfer of objects to and from S3,
+   S3TransferManager is the preferred solution. It is easier and more intuitive
+   than using the S3 client. In the majority of situations, it is more
+   performant.
+1. S3TransferManager provides a truly asynchronous, non-blocking API that
+   conforms to the norms present in the rest of the SDK.
+1. S3TransferManager makes efficient use of system resources.
+1. S3TransferManager supplements rather than replaces the lower level S3 client.
+
+# Non Project Goals
+
+1. Ability to use the blocking, synchronous client.
+
+   Using a blocking client would severely impede the ability to deliver on goals
+   #2 and #3.
+
+# Customer-Requested Changes from 1.11.x
+
+* S3TransferManager supports progress listeners that are easier to use.
+
+  Ref: https://github.com/aws/aws-sdk-java-v2/issues/37#issuecomment-316218667
+
+* S3TransferManager provides bandwidth limiting of uploads and downloads.
+
+  Ref: https://github.com/aws/aws-sdk-java/issues/1103
+
+* The size of resources used by Transfermanager and configured by the user
+  should not affect its stability.
+
+  For example, the configured size of a threadpool should be irellevant to its
+  ability to successfuly perform an operation.
+
+  Ref: https://github.com/aws/aws-sdk-java/issues/939
+
+* S3TransferManager supports parallel downloads of any object.
+
+  Any object stored in S3 should be downloadable in multiple parts
+  simultaneously, not just those uploaded using the Multipart API.
+
+* S3TransferManager has the ability to upload to and download from a pre-signed
+  URL.
+
+* S3TransferManager allows uploads and downloads from and to memory.
+
+  Ref: https://github.com/aws/aws-sdk-java/issues/474
+
+* Ability to easily use canned ACL policies with all transfers to S3.
+
+  Ref: https://github.com/aws/aws-sdk-java/issues/1207
+
+* Trailing checksums for parallel uploads and downloads.