Added README.

README.md

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+# GitHub Actions Tester
+
+This Node application exposes an executable that allows you to test
+your GitHub workflow locally using Docker.
+
+
+## Installation
+
+Install using [NPM](https://npmjs.com):
+
+```
+npm i -g gha
+```
+
+
+### Usage
+
+```
+Usage: gha [options]
+
+Options:
+  -V, --version      output the version number
+  -f <workflowfile>  Set workflow file path, defaults to .github/main.workflow
+  -e <event>         Set event, defaults to push
+  -h, --help         output usage information
+```