Add custom field autocomplete documentation

Author: anttiai

README.md

@@ -57,18 +57,33 @@ client.search();
 ```
 
 #### Fetch search suggestions
+Search suggestions are keywords and search phrases that real users have used in your search. Configure Search 
+suggestions on AddSearch Dashboard before using this function.
 ```js
-// Configure Search suggestions on AddSearch Dashboard first
 // Get suggestions starting with a specific prefix
 client.suggestions('a', callback);
 ```
 
-#### Number of search suggestions
+#### Set the number of search suggestions to fetch
 ```js
 // Number of search suggestions to fetch (default 10)
 client.setSuggestionsSize(20);
 ```
 
+#### Fetch custom field autocompletion
+Custom fields autocomplete can be used for predictive search. For example, product names or categories can be
+suggested as the keyword is being typed in.
+```js
+// Fetch custom field values starting with a specific prefix. In this example, results could be "adidas, apple, azure"
+client.autocomplete('custom_fields.brand', 'a', callback);
+```
+
+#### Set the number of custom field autocompletion results to fetch
+```js
+// Number of autocompletion results to fetch (default 10)
+client.setAutocompleteSize(20);
+```
+
 #### Search with fuzzy matching
 ```js
 // Enable/disable fuzzy matching. Possible values true/false/"auto" (default: "auto")