Add README describing the system and environment variables

The README includes information about why this is being written as well
as the environment variables necessary to execute it.

Closes #1

LICENSE

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+The MIT License (MIT)
+
+Copyright (c) 2020 Bryan J Swift
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+

README.md

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+# Email Service
+
+The goal of the email service is to enable the decoupling of triggering email
+notifications from the work of transmitting email to a third party service and
+receiving callbacks from the services.
+
+## Queue
+
+The service polls an Amazon SQS queue looking for messages. The SQS messages
+are expected to have a JSON body containing an `email_id` key. The `email_id`
+is used to look up the email information in a database.
+
+## Database
+
+The `email_id` from the queue message is used to look up a record in an Amazon
+DynamoDB table. The keys and values in the DynamoDB row are mapped to a
+structure representing the data a third party email sending service needs to
+transmit the message.
+
+## Email Delivery Service(s)
+
+No third party email sending service(s) are implemented yet.
+
+## Environment Variables
+
+AWS credentials are read from the environment by default. The email service
+relies on [rusoto][rusoto] to access AWS. Rusoto provides a [`credential`
+crate][credential] which is used to determine how AWS resources will be
+accessed. The [`ChainProvider`][credential_chain_provider] currently defines
+how AWS credentials are read.
+
+[rusoto]: https://github.com/rusoto/rusoto
+[credential]: https://crates.io/crates/rusoto_credential
+[credential_chain_provider]: https://docs.rs/rusoto_credential/0.43.0-beta.1/rusoto_credential/struct.ChainProvider.html
+
+- `REGION` defines the AWS region where the SQS queue and DynamoDB table are
+  located.
+- `QUEUE_URL` defines the SQS queue polled for messages.
+- `DRY_RUN` when defined as the string `true` the queue will only be polled a
+  single time and no email information will be transmitted to the email sending
+  service(s).
+
+## Development
+
+This has been developed on MacOS using [Rust v1.41.0][rust-stable]. See [Rust
+install docs][rust-install] for more detailed setup information.
+
+[rust-stable]: https://www.rust-lang.org
+[rust-install]: https://www.rust-lang.org/tools/install
+
+### Prerequisites
+
+`cargo build` will download and install the dependencies specified in
+[`Cargo.toml`](Cargo.toml). In order to successfully test the application it
+will need access to an SQS queue and if the queue contains any messages it will
+attempt to access the DynamoDB table to retrieve records it is necessary to
+have:
+
+- An Amazon SQS queue URL
+- A DynamoDB table name
+- AWS credentials in the environment that can access both
+
+### Test
+
+```shell
+cargo test
+```
+
+### Run
+
+```
+DRY_RUN="true" \
+  AWS_REGION="us-east-1" \
+  QUEUE_URL="https://sqs.us-east-1.amazonaws.com/<account_id>/<queue_name>" \
+  cargo run
+```
+
+### Build
+
+```shell
+cargo build --release
+```
+
+## License
+
+This Paw Extension is released under the [MIT License](LICENSE). Feel free to
+fork, and modify!
+
+Copyright © 2020 Bryan J Swift