Update docs according to feature introduced in latest webpack.

Author: EugeneHlushko

src/content/configuration/output.md

@@ -236,8 +236,16 @@ The prefix length of the hash digest to use, defaults to `20`.
 
 ## `output.hashFunction`
 
+`string|function`
+
 The hashing algorithm to use, defaults to `'md5'`. All functions from Node.JS' [`crypto.createHash`](https://nodejs.org/api/crypto.html#crypto_crypto_createhash_algorithm) are supported.
+Since v4.0.0-alpha2 `hashFunction` can now be a constructor to a custom hash function. You can provide a non-crypto hash function for performance reasons.
+
+``` js
+hashFunction: require('metrohash').MetroHash64
+```
 
+Make sure that hashing function will have `update` and `digest` methods available.
 
 ## `output.hashSalt`