Add README

README.md

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+# reviewdog/action-setup
+
+[![Test](https://github.com/reviewdog/action-setup/workflows/Test/badge.svg)](https://github.com/reviewdog/action-setup/actions?query=workflow%3ATest)
+[![reviewdog](https://github.com/reviewdog/action-setup/workflows/reviewdog/badge.svg)](https://github.com/reviewdog/action-setup/actions?query=workflow%3Areviewdog)
+[![release](https://github.com/reviewdog/action-setup/workflows/release/badge.svg)](https://github.com/reviewdog/action-setup/actions?query=workflow%3Arelease)
+[![GitHub release (latest SemVer)](https://img.shields.io/github/v/release/reviewdog/action-setup?logo=github&sort=semver)](https://github.com/reviewdog/action-setup/releases)
+[![action-bumpr supported](https://img.shields.io/badge/bumpr-supported-ff69b4?logo=github&link=https://github.com/haya14busa/action-bumpr)](https://github.com/haya14busa/action-bumpr)
+
+This action installs [reviewdog](https://github.com/reviewdog/reviewdog).
+
+## Input
+```yaml
+inputs:
+  reviewdog_version:
+    description: 'reviewdog version. [latest,nightly,vX.Y.Z]'
+    default: 'latest'
+```
+
+## Usage
+
+### Latest
+```yaml
+steps:
+- uses: reviewdog/action-setup@v1
+- run: reviewdog -version
+```
+
+### Specify reviewdog version
+```yaml
+steps:
+- uses: reviewdog/action-setup@v1
+  with:
+    reviewdog_version: v0.10.1
+- run: reviewdog -version
+```
+
+### Nightly
+```yaml
+steps:
+- uses: reviewdog/action-setup@v1
+  with:
+    reviewdog_version: nightly
+- run: reviewdog -version
+```