Add basic README for aws-sdk-signers (smithy-lang#473)

Author: nateprewitt

packages/aws-sdk-signers/README.md

@@ -0,0 +1,31 @@
+## AWS SDK Signers for Python
+
+AWS SDK Signers for Python provides stand-alone signing functionality. This enables users to
+create standardized request signatures (currently only SigV4) and apply them to
+common HTTP utilities like AIOHTTP, Curl, Postman, Requests and urllib3.
+
+This project is currently in an **Alpha** phase of development. There likely
+will be breakages and redesigns between minor patch versions as we collect
+user feedback. We strongly recommend pinning to a minor version and reviewing
+the changelog carefully before upgrading.
+
+## Getting Started
+
+Currently, the `aws-sdk-signers` module provides two high level signers,
+`AsyncSigV4Signer` and `SigV4Signer`.
+
+Both of these signers takes three inputs to their primary `sign` method.
+
+* A [**SigV4SigningProperties**](https://github.com/smithy-lang/smithy-python/blob/3d205be8ece1c5f4c992a29ce9757c5562e59322/packages/aws-sdk-signers/src/aws_sdk_signers/signers.py#L43-L49) object defining:
+  * The service for the request,
+  * The intended AWS region (e.g. us-west-2),
+  * An optional date that will be auto-populated with the current time if not supplied,
+  * An optional boolean, payload_signing_enabled to toggle payload signing. True by default.
+  * An optional boolean, content_checksum_enabled, to include the x-amz-content-sha256 header. True by default.
+  * An optional boolean, uri_encode_path, to toggle double-encoding the URI path. True by default.
+* An [**AWSRequest**](https://github.com/smithy-lang/smithy-python/blob/3d205be8ece1c5f4c992a29ce9757c5562e59322/packages/aws-sdk-signers/src/aws_sdk_signers/_http.py#L335), similar to the [AWSRequest object](https://github.com/boto/botocore/blob/7d197f9e1fe903ba3badee62a1ecac916ba2cfb5/botocore/awsrequest.py#L433) from boto3 or the [Request object](https://requests.readthedocs.io/en/latest/api/#requests.Request) from Requests.
+* An [**AWSCredentialIdentity**](https://github.com/smithy-lang/smithy-python/blob/3d205be8ece1c5f4c992a29ce9757c5562e59322/packages/aws-sdk-signers/src/aws_sdk_signers/_identity.py#L11), a dataclass holding standard AWS credential information.
+
+## License
+
+This project is licensed under the Apache-2.0 License.