docs: update README [ci skip]

Author: kei711

README.md

@@ -1,17 +1,40 @@
 # Docbase Url Expander
 
+Slack bot for unfurling [Docbase](https://docbase.io) URLs.
+
+[![Latest Version](https://img.shields.io/github/v/release/kei711/docbase-url-expander)](https://github.com/kei711/docbase-url-expander/releases)
 [![Build](https://github.com/kei711/docbase-url-expander/actions/workflows/main.yml/badge.svg)](https://github.com/kei711/docbase-url-expander/actions/workflows/main.yml)
 
 [GitHub Container Registry](https://github.com/kei711/docbase-url-expander/pkgs/container/docbase-url-expander)
 
-## Prerequisite
+## Usage
+
+```shell
+# copy and edit the .env file
+cp .env.sample .env
+docker run --env-file .env --init --rm ghcr.io/kei711/docbase-url-expander:latest
+```
+
+### Environment variables
+
+- SLACK_BOT_TOKEN
+  - Bot token strings begin with `xoxb-`
+- SLACK_APP_TOKEN
+  - App token strings begin with `xapp-`
+- DOCBASE_TEAM_NAME
+  - `https://<DOCBASE_TEAM_NAME>.docbase.io`
+- DOCBASE_ACCESS_TOKEN
+  - Access token of Docbase
+  - see [アクセストークン作成方法](https://help.docbase.io/posts/45703#%E3%82%A2%E3%82%AF%E3%82%BB%E3%82%B9%E3%83%88%E3%83%BC%E3%82%AF%E3%83%B3%E4%BD%9C%E6%88%90%E6%96%B9%E6%B3%95)
+
+## For local development
+
+### Prerequisite
 
 - Node.js 16.x
 - Yarn 1.x
 - direnv
 
-## Commands
-
 ### Setup
 
 ```shell