docs(README): clarify to set allChunks option when using CommonsChunk…

…Plugin (#476)

README.md

@@ -69,7 +69,7 @@ new ExtractTextPlugin(options: filename | object)
 |:--:|:--:|:----------|
 |**`id`**|`{String}`|Unique ident for this plugin instance. (For advanced usage only, by default automatically generated)|
 |**`filename`**|`{String\|Function}`|Name of the result file. May contain `[name]`, `[id]` and `[contenthash]`|
-|**`allChunks`**|`{Boolean}`|Extract from all additional chunks too (by default it extracts only from the initial chunk(s))|
+|**`allChunks`**|`{Boolean}`|Extract from all additional chunks too (by default it extracts only from the initial chunk(s))<br />When using `CommonsChunkPlugin` and there are extracted chunks (from `ExtractTextPlugin.extract`) in the commons chunk, `allChunks` **must** be set to `true`|
 |**`disable`**|`{Boolean}`|Disables the plugin|
 |**`ignoreOrder`**|`{Boolean}`|Disables order check (useful for CSS Modules!), `false` by default|