docs: document unit, integration, and installation (#4)

README.md

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+# Magento 2 GitHub Actions
+
+Opinionated Github Actions and Workflows to make building, testing, and maintaining Magento 2 Modules easier.
+
+* [README if you are new to Github Actions.](https://docs.github.com/en/actions/learn-github-actions/understanding-github-actions#the-components-of-github-actions)
+* [What is a workflow?](https://docs.github.com/en/actions/learn-github-actions/understanding-github-actions#workflows)
+* [What is an action?](https://docs.github.com/en/actions/learn-github-actions/understanding-github-actions#actions)
+
+## Workflows
+
+| Workflow Name                                            | Description                                                            |
+| -------------------------------------------------------- | ---------------------------------------------------------------------- |
+| [Integration Test](./.github/workflows/integration-README.md) | A Github Workflow that runs the Integration Tests of a Magento Package |
+
+## Actions
+
+| Action Name                                      | Description                                                        |
+| ------------------------------------------------ | ------------------------------------------------------------------ |
+| [Unit Test](./unit-test/README.md)               | A Github Action that runs the Unit Tests a Magento Package         |
+| [Installation Test](installation-test/README.md) | A Github Action that tests the installability of a Magento Package |
+
+### Action or Workflow?
+
+If you're just getting started with your Magento package on Github, you probably want to use the workflows.
+
+If you're looking to customize further, or you need to support a broader range of Magento versions, then you probably want an action.