update shortcuts in readme

* Ctrl+A可选中界面中所有文本块

* Ctrl+F查找当前页, Ctrl+G全局查找

* 0-9调整嵌字/原图透明度

* 文本编辑下```Ctrl+B```加粗, ```Ctrl+U```下划线, ```Ctrl+I```斜体

<img src="doc/src/configpanel.png">  

* ```Ctrl++```/```Ctrl+-``` (Also ```Ctrl+Shift+=```) to resize image.

* ```Ctrl+G```/```Ctrl+F``` to search globally/in current page.

* ```0-9``` to adjust opacity of lettering layer

* For text editing: bold - ```Ctrl+B```, underline - ```Ctrl+U```, Italics - ```Ctrl+I``` 

<img src="doc/src/configpanel.png">