Crop spec and documentation update

README.md

@@ -56,10 +56,11 @@ screenshot = Gastly.screenshot('http://google.com', selector: '#hplogo', timeout
 image = screenshot.capture
 ```
 
-You can resize or change the format of the screenshot:
+You can resize, crop or change the format of the screenshot:
 
 ```ruby
 image = screenshot.capture
+image.crop(width: 250, height: 250, x: 10, y: 0) # Crop with an offset
 image.resize(width: 110, height: 110) # Creates a preview of the web page
 image.format('png')
 image.save('output.png')

spec/gastly/image_spec.rb

@@ -12,6 +12,14 @@
     end
   end
 
+  context '#crop' do
+    it 'invokes method #crop with arguments' do
+      width, height, x, y = 100, 100, 0, 0
+      expect_any_instance_of(MiniMagick::Image).to receive(:crop).with("#{width}x#{height}+#{x}+#{y}")
+      subject.crop(width: 100, height: 100, x: 0, y: 0)
+    end
+  end
+
   context '#format' do
     it 'invokes method #format' do
       ext = 'png'