Add documentation for [JS-SDK] ParseQuery.withCount (#644)

* Documentation for ParseQuery.withCount

See:
parse-community/Parse-SDK-JS#865
parse-community/Parse-SDK-JS#868

* added disclaimer

* remove extra line breaks

* Update _includes/js/queries.md

Co-Authored-By: Tom Fox <tomfox@surprises.io>

* Update _includes/js/queries.md

Co-Authored-By: Tom Fox <tomfox@surprises.io>

* simpler

Author: BufferUnderflower

_includes/js/queries.md

@@ -57,6 +57,27 @@ You can skip the first results by setting `skip`. In the old Parse hosted backen
 query.skip(10); // skip the first 10 results
 ```
 
+If you want to know the total number of rows in a table satisfying your query, for  e.g. pagination purposes - you can use `withCount`. 
+
+**Note:** Enabling this flag will change the structure of response, see the example below.
+
+Let's say you have 200 rows in a table called `GameScore`:
+
+```javascript
+const GameScore = Parse.Object.extend("GameScore");
+const query = new Parse.Query(GameScore);
+
+query.limit(25);
+
+const results = await query.find(); // [ GameScore, GameScore, ...]
+
+// to include count:
+query.withCount();
+const response = await query.find(); // { results: [ GameScore, ... ], count: 200 }
+```
+⚠️ Сount operations can be slow and expensive.
+> If you only want to get the count without objects - use [Counting Objects](#counting-objects).
+
 For sortable types like numbers and strings, you can control the order in which results are returned:
 
 ```javascript