Update README

Makefile

@@ -16,3 +16,6 @@ clean:
 
 lint:
 	golangci-lint run
+
+test:
+	go test -v ./...

README.md

@@ -1,10 +1,90 @@
 # ECS Toolkit
 
+## Configuration
+
+### AWS Credentials
+
+The AWS SDK used internally uses its default credential chain to find AWS
+credentials. The SDK detects and uses the built-in providers automatically,
+without requiring manual configuration. For example, if you use IAM roles for
+Amazon EC2 instances, it automatically use the instance’s credentials. For more
+information on setting up AWS credentials, see "[Configuring the AWS
+CLI][aws-configuration-cli]" and "[Configuring the AWS SDK for Go
+V2][aws-configuration-sdk]".
+
+### Config File
+
+The config file for an application would look like what you see below:
+
+```yaml
+---
+version: v1
+cluster: <name of cluster>
+tasks:
+  - name: <task-definition-1-name>
+    containers:
+      - <container-1-name>
+  - name: <task-definition-2-name>
+    containers:
+      - <container-1-name>
+      - <container-2-name>
+      # add as many containers as needed
+  # add as many tasks as needed
+services:
+  - name: <service-1-name>
+    containers:
+      - <container-1-name>
+  - name: <service-2-name>
+    containers:
+      - <container-1-name>
+  - name: <service-3-name>
+    containers:
+      - <container-1-name>
+  # add as many services as needed
+```
+
+## Usage
+
+By default, the tool will use the config file located at `.ecs-toolkit.yml` but
+you can specify an alternative using the `--config` flag. With that, it's easy
+to deploy your application using the `deploy` command:
+
+```console
+$ ecs-toolkit -l debug deploy --image-tag=49779134ca1dcef21f0b5123d3d5c2f4f47da650
+```
+
+## Development
+
+Below instructions are only necessary if you intend to work on the source code.
+For normal usage the above instructions should do.
+
+### Requirements
+
+1. Ensure that you have a [properly configured][golang-quickstart] Go workspace.
+
+### Building
+
+1. Clone the repository.
+2. Fetch the dependencies with `go get -v github.com/shipatlas/ecs-toolkit`.
+4. Install application dependencies via `make dependencies` (they'll be placed
+   in `./vendor`).
+5. Build and install the binary with `make build`.
+6. Run the command e.g. `./bin/ecs-toolkit help` as a basic test.
+
+### Testing
+
+1. Install the `golangci-lint`, [see instructions here][golangci-lint-install].
+2. Run linter using `make lint` and test using `make test`.
+
 ## License
 
 [ShipAtlas][shipatlas] © 2022. The [MIT License bundled therein][license] is a
 permissive license that is short and to the point. It lets people do anything
 they want as long as they provide attribution and waive liability.
 
+[aws-configuration-cli]: https://docs.aws.amazon.com/cli/latest/userguide/cli-configure-quickstart.html
+[aws-configuration-sdk]: https://aws.github.io/aws-sdk-go-v2/docs/configuring-sdk/
+[golang-quickstart]: https://go.dev/doc/tutorial/getting-started
+[golangci-lint-install]: https://golangci-lint.run/usage/install/
 [license]: https://raw.githubusercontent.com/shipatlas/ecs-toolkit/main/LICENSE
 [shipatlas]: https://www.shipatlas.dev