doc: write release process

ref: #560

.github/CONTRIBUTING.md

@@ -0,0 +1,14 @@
+# Contributing
+
+TBD for other type of contribution.
+
+## Release Pull Request (Release Process)
+You can create a Pull Request to release a new version.
+
+1. Update [CHANGELOG.md](../CHANGELOG.md). Copy unreleased section and create a new version section.
+2. Create a Pull Request.
+3. Attach a label [`bump:patch`, `bump:minor`, or `bump:major`]. reviewdog uses [haya14busa/action-bumpr](https://github.com/haya14busa/action-bumpr).
+4. [The release workflow](./workflows/release.yml) automatically tag a
+   new version depending on the label and create a new release on merging the
+   Pull Request.
+5. Check the release workflow and verify the release. [![release](https://github.com/reviewdog/reviewdog/workflows/release/badge.svg)](https://github.com/reviewdog/reviewdog/actions?query=workflow%3Arelease)

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,3 @@
+
+- [ ] Updated Unreleased section in [CHANGELOG](https://github.com/reviewdog/reviewdog/blob/master/CHANGELOG.md) or it's not notable changes.
+

README.md

@@ -30,6 +30,9 @@
   <a href="https://github.com/reviewdog/reviewdog/actions?query=workflow%3Areviewdog+event%3Apush+branch%3Amaster">
     <img alt="reviewdog" src="https://github.com/reviewdog/reviewdog/workflows/reviewdog/badge.svg?branch=master&event=push">
   </a>
+  <a href="https://github.com/reviewdog/reviewdog/actions?query=workflow%3Arelease">
+    <img alt="release" src="https://github.com/reviewdog/reviewdog/workflows/release/badge.svg">
+  </a>
   <a href="https://travis-ci.org/reviewdog/reviewdog"><img alt="Travis Status" src="https://img.shields.io/travis/reviewdog/reviewdog/master.svg?label=Travis&logo=travis"></a>
   <a href="https://circleci.com/gh/reviewdog/reviewdog"><img alt="CircleCI Status" src="http://img.shields.io/circleci/build/github/reviewdog/reviewdog/master.svg?label=CircleCI&logo=circleci"></a>
   <a href="https://codecov.io/github/reviewdog/reviewdog"><img alt="Coverage Status" src="https://img.shields.io/codecov/c/github/reviewdog/reviewdog/master.svg?logo=codecov"></a>