Add README

Author: tlinhart

README.md

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+# urllib3 SigV4
+
+Extension to [urllib3](https://github.com/urllib3/urllib3) adding support for
+signing the requests with AWS Signature Version 4. It uses the
+[Boto3](https://github.com/boto/boto3) library for handling the AWS credentials
+and the actual signing process.
+
+## Installation
+
+Use `pip` to install the package:
+
+```bash
+pip install urllib3_sigv4
+```
+
+## Usage
+
+This library provides a drop-in replacement for two main components of urllib3,
+the [`PoolManager`](https://urllib3.readthedocs.io/en/stable/reference/urllib3.poolmanager.html)
+class and the top-level [`request`](https://urllib3.readthedocs.io/en/stable/reference/urllib3.request.html)
+method. It adds a new optional parameter which determines if and how the
+requests should be signed.
+
+### Creating a Signer
+
+First, create an instance of the `SigV4RequestSigner` class which defines the
+parameters for request signing:
+
+```python
+from urllib3_sigv4 import SigV4RequestSigner
+
+signer = SigV4RequestSigner(
+    "lambda",
+    region="eu-central-1",
+    access_key="AKIAIOSFODNN7EXAMPLE",
+    secret_key="wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY"
+)
+```
+
+The first parameter is mandatory and identifies the AWS service we want to make
+requests to (AWS Lambda in this case). The `region`, `access_key` and
+`secret_key` parameters are optional and will be inferred from the environment
+if not passed (via the default Boto3 session, see
+[here](https://boto3.amazonaws.com/v1/documentation/api/latest/guide/session.html)
+and [here](https://boto3.amazonaws.com/v1/documentation/api/latest/guide/credentials.html)
+for more details).
+
+### Making Requests
+
+To make signed requests to an AWS service, pass the signer instance via the
+`signer` parameter when creating the `PoolManager`:
+
+```python
+from urllib3_sigv4 import PoolManager, SigV4RequestSigner
+
+signer = SigV4RequestSigner("lambda")
+http = PoolManager(signer=signer)
+
+response = http.request(
+    "POST",
+    "https://my-lambda-url-id.lambda-url.eu-central-1.on.aws",
+    json={"name": "John Doe", "age": 30}
+)
+print(response.json())
+```
+
+You can also provide the signer in individual `request` method calls to
+override the default behavior:
+
+```python
+from urllib3_sigv4 import PoolManager, SigV4RequestSigner
+
+signer = SigV4RequestSigner("lambda")
+http = PoolManager()
+
+# The same as when using urllib3's PoolManager.
+response = http.request("GET", "https://httpbin.org/get")
+print(response.json())
+
+# This request will be signed.
+response = http.request(
+    "POST",
+    "https://my-lambda-url-id.lambda-url.eu-central-1.on.aws",
+    json={"name": "John Doe", "age": 30},
+    signer=signer
+)
+print(response.json())
+```
+
+You can also use a convenience top-level `request` method which uses a
+module-global `PoolManager` instance:
+
+```python
+from urllib3_sigv4 import SigV4RequestSigner, request
+
+signer = SigV4RequestSigner("lambda")
+
+response = request(
+    "POST",
+    "https://my-lambda-url-id.lambda-url.eu-central-1.on.aws",
+    json={"name": "John Doe", "age": 30},
+    signer=signer
+)
+print(response.json())
+```
+
+## Reference
+
+- https://docs.aws.amazon.com/IAM/latest/UserGuide/reference_aws-signing.html
+- https://github.com/aws-samples/sigv4-signing-examples
+- https://github.com/awslabs/aws-sdk-python-signers