Update README.md

Author: norio-nomura

README.md

@@ -1,3 +1,35 @@
-# action-swiftlint
+# GitHub Action for SwiftLint
 
-A description of this package.
+This Action executes [SwiftLint](https://github.com/realm/SwiftLint) and generates annotations from SwiftLint Violations by using [GitHub Checks API](https://blog.github.com/2018-05-07-introducing-checks-api/).
+
+## Usage
+
+An example workflow to executing SwiftLint follows:
+
+```hcl
+workflow "Execute SwiftLint" {
+  on = "push"
+  resolves = ["swiftlint"]
+}
+
+action "swiftlint" {
+  uses = "norio-nomura/action-swiftlint@master"
+  secrets = ["GITHUB_TOKEN"]
+}
+```
+
+## Secrets
+
+- Specifying `GITHUB_TOKEN` to `secrets` is required to using [Check Run APIs](https://developer.github.com/v3/checks/runs/) for generating annotations from SwiftLint Violations.
+
+## Example
+[Here](https://github.com/norio-nomura/test-action-swiftlint/pull/1/files#annotation_9749095) is an example that actually works.
+![screenshot](screenshot.png)
+
+## Author
+
+Norio Nomura
+
+## License
+
+[MIT](LICENSE)