Make API port configurable and add documentation

vue/README.md

@@ -10,6 +10,14 @@ npm install
 npm run serve
 ```
 
+Change the port Vue binds to via the `--port` parameter.
+Use the `VUE_APP_API_PORT` environment variable to point Vue to the JHipster API port.
+For example, to specify both settings:
+
+```
+VUE_APP_API_PORT=45678 npm run serve -- --port=2345
+```
+
 ### Compiles and minifies for production
 ```
 npm run build