feat:add docs

docs/guide/contextmenu-custom.md

@@ -0,0 +1,20 @@
+# 自定义右键菜单
+
+## 使用方式
+
+```javascript
+import Editor from "@hufe921/canvas-editor"
+
+const instance = new Editor(container, <IElement[]>data, options)
+instance.register.contextMenuList([
+    {
+      isDivider?: boolean;
+      icon?: string;
+      name?: string;
+      shortCut?: string;
+      when?: (payload: IContextMenuContext) => boolean;
+      callback?: (command: Command, context: IContextMenuContext) => any;
+      childMenus?: IRegisterContextMenu[];
+    }
+  ])
+```