docs: add info about option "shouldCluster" (#67)

README.md

@@ -89,10 +89,10 @@ looksSame('image1.png', 'image2.png', {ignoreAntialiasing: true, antialiasingTol
 Looksame returns information about diff bounds. It returns only first pixel if you passed `stopOnFirstFail` option with `true` value. The whole diff area would be returned if `stopOnFirstFail` option is not passed or it's passed with `false` value.
 
 ### Getting diff clusters
-Looksame returns diff bounds divided into clusters. You can pass clusters size using `clustersSize` option.
+Looksame returns diff bounds divided into clusters if option `shouldCluster` passed with `true` value. Moreover you can pass clusters size using `clustersSize` option.
 
 ```javascript
-looksSame('image1.png', 'image2.png', {stopOnFirstFail: false}, function(error, {equal, diffBounds, diffClusters}) {
+looksSame('image1.png', 'image2.png', {shouldCluster: true, clustersSize: 10}, function(error, {equal, diffBounds, diffClusters}) {
     // {
     //     equal: false,
     //     diffBounds: {left: 10, top: 10, right: 20, bottom: 20}