docs: add release guide

Author: incendial

RELEASE_GUIDE.md

@@ -0,0 +1,36 @@
+# Release guide
+
+## Changelog updates
+
+* each entry should end with a `.`
+* all updated entities has a link pointing to the website (ex. rule update, metric update)
+
+## Check and bump version
+
+* the version should be updated in:
+  * ./CHANGELOG.md
+  * ./CONTRIBUTING.md
+  * ./pubspec.yaml
+  * ./README.md
+  * ./lib/src/version.dart
+  * ./tools/analyzer_plugin/pubspec.yaml
+  * ./website/docs/getting-started/installation.md
+
+## Publishing
+
+* commit and push all changes to the master branch
+* create a git tag with a new version name
+* run `dart pub publish --dry-run.`. Only one warning is allowed:
+
+  ```bash
+  Rename the top-level "tools" directory to "tool".
+    The Pub layout convention is to use singular directory names.
+    Plural names won't be correctly identified by Pub and other tools.
+    See https://dart.dev/tools/pub/package-layout.
+  ```
+
+* run `dart pub publish`
+* create a [new draft release on pub](https://github.com/dart-code-checker/dart-code-metrics/releases)
+  * select the newly created tag
+  * generate release notes (clean them from the dependabot updates)
+  * publish release