Import Introduction from README

src/introduction.md

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 # Introduction
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+![act-logo](https://github.com/nektos/act/wiki/img/logo-150.png)
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+> "Think globally, `act` locally"
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+Run your [GitHub Actions](https://developer.github.com/actions/) locally! Why would you want to do this? Two reasons:
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+- **Fast Feedback** - Rather than having to commit/push every time you want to test out the changes you are making to your `.github/workflows/` files (or for any changes to embedded GitHub actions), you can use `act` to run the actions locally. The [environment variables](https://help.github.com/en/actions/configuring-and-managing-workflows/using-environment-variables#default-environment-variables) and [filesystem](https://help.github.com/en/actions/reference/virtual-environments-for-github-hosted-runners#filesystems-on-github-hosted-runners) are all configured to match what GitHub provides.
+- **Local Task Runner** - I love [make](<https://en.wikipedia.org/wiki/Make_(software)>). However, I also hate repeating myself. With `act`, you can use the GitHub Actions defined in your `.github/workflows/` to replace your `Makefile`!
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+# How Does It Work?
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+When you run `act` it reads in your GitHub Actions from `.github/workflows/` and determines the set of actions that need to be run. It uses the Docker API to either pull or build the necessary images, as defined in your workflow files and finally determines the execution path based on the dependencies that were defined. Once it has the execution path, it then uses the Docker API to run containers for each action based on the images prepared earlier. The [environment variables](https://help.github.com/en/actions/configuring-and-managing-workflows/using-environment-variables#default-environment-variables) and [filesystem](https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners#file-systems) are all configured to match what GitHub provides.
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+Let's see it in action with a [sample repo](https://github.com/cplee/github-actions-demo)!
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+![Demo](https://github.com/nektos/act/wiki/quickstart/act-quickstart-2.gif)