Add notes on Web Components (#831)

javascript/README.md

@@ -177,6 +177,20 @@ import 'budoux/module/webcomponents/budoux-zh-hant';
 import 'budoux/module/webcomponents/budoux-th';
 ```
 
+**Note:** BudouX Web Components directly manipulate the input HTML content
+instead of outputting the result to a shadow DOM. This design was chosen because
+the goal of BudouX Web Components is to simply insert zero-width spaces (ZWSPs)
+into the content, and isolating the style from the rest of the document could
+introduce unexpected side effects for developers.
+
+Consequently, cloning or editing the element might lead to duplicated ZWSPs
+between phrases. This is because BudouX Web Components cannot distinguish
+between characters that originate in the source and those that are inserted by
+BudouX itself once connected to the document. Duplicating ZWSPs will not cause
+any severe problems in controlling line breaks, and they are invisible anyway,
+but this is the reason we do not support other separator characters for these
+components.
+
 ### CLI
 
 You can also format inputs on your terminal with `budoux` command.