docs: document dumi dev server port

Author: afc163

README.md

@@ -104,6 +104,8 @@ npm install
 npm start
 ```
 
+The dumi site runs at `http://localhost:8000` by default.
+
 ```bash
 npm test
 npm run tsc

README.zh-CN.md

@@ -104,6 +104,8 @@ npm install
 npm start
 ```
 
+dumi 站点默认运行在 `http://localhost:8000`。
+
 ```bash
 npm test
 npm run tsc