update shortcuts doc

src/resources/docs/shortcuts_en.md

@@ -51,6 +51,8 @@ Backspace. Delete a character backward.
 Delete all the characters from current cursor to the first space backward.
 - `Ctrl+U`  
 Delete all the characters from current cursor to the beginning of current line.
+- `Ctrl+Alt+<Num>`  
+Insert title at level `<Num>`. `<Num>` should be 1 to 6. Currently selected text will be changed to title if exist.
 - `Tab`/`Shift+Tab`  
 Increase or decrease the indentation. If any text is selected, the indentation will operate on all these selected lines.
 - `Shift+Left`, `Shift+Right`, `Shift+Up`, `Shift+Down`  

src/resources/docs/shortcuts_zh.md

@@ -51,6 +51,8 @@
 删除光标位置向后到第一个空白字符之间的所有字符。
 - `Ctrl+U`  
 删除光标位置到行首的所有字符。
+- `Ctrl+Alt+<Num>`  
+插入级别为`<Num>`的标题。`<Num>`应该是1到6的一个数字。如果已经选择文本，则将当前选择文本改为标题。
 - `Tab`/`Shift+Tab`  
 增加或减小缩进。如果已经选择文本，则对所有选择的行进行缩进操作。
 - `Shift+Left`, `Shift+Right`, `Shift+Up`, `Shift+Down`