refine Markdown guide for Graphviz

changes.md

@@ -2,7 +2,7 @@
 ## v1.15
 - Support **PlantUML** and **Graphviz**;
 - **In-Place Preview** for MathJax, PlantUML, Graphviz, and Flowchart.js;
-- **Live Preview** for diagrams and MathJax via `Ctrl+E I`;
+- **Live Preview** for diagrams via `Ctrl+E I`;
 - Restore cursor position when recovering pages at startup;
 - UniversalEntry
     - Ctrl+I to expand/collapse current item;

src/resources/docs/markdown_guide_en.md

@@ -139,6 +139,12 @@ VNote supports [PlantUML](http://plantuml.com/) to draw UML diagrams. You should
     @enduml
     ```
 
+#### Graphviz
+
+> You need to enable Graphviz in the settings.
+
+VNote supports [Graphviz](http://www.graphviz.org/) to draw diagrams. You should use `dot` specified as the language of the fenced code block and write the definition of your diagram within it.
+
 ### Math Formulas
 
 > You need to enable MathJax in the `Markdown` menu.

src/resources/docs/markdown_guide_zh.md

@@ -140,6 +140,12 @@ VNote支持 [PlantUML](http://plantuml.com/) 来实现UML图表。您需要使
     @enduml
     ```
 
+#### Graphviz
+
+> 需要在设置中启用Graphviz。
+
+VNote支持 [Graphviz](http://www.graphviz.org/) 来绘制图表。您需要使用代码块，并标明语言为`dot`，然后在代码块里面定义图表。
+
 ### 数学公式
 
 > 需要在`Markdown`菜单中启用MathJax。