fix: add cacheIdentifier to documentation

Author: sokra

README.md

@@ -48,6 +48,7 @@ module.exports = {
 |Name|Type|Default|Description|
 |:--:|:--:|:-----:|:----------|
 |**`cacheDirectory`**|`{String}`|`path.resolve('.cache-loader')`|Provide a cache directory where cache items should be stored|
+|**`cacheIdentifier`**|`{String}`|`cache-loader:{version} {process.env.NODE_ENV}`|Provide an invalidation identifier which is used to generate the hashes. You can use it for extra dependencies of loaders.|
 
 <h2 align="center">Examples</h2>