update podspec for UIGlossyButton that all subspecs are opt-in

UIGlossyButton/0.0.7/UIGlossyButton.podspec

@@ -8,16 +8,18 @@ Pod::Spec.new do |s|
 
   s.platform      = :ios
   s.source        = { :git => 'https://github.com/waterlou/UIGlossyButton.git', :tag => 'v0.0.7' }
-  s.preferred_dependency = 'Core'
+  ## s.preferred_dependency = 'Core'
 
   s.frameworks    = 'UIKit', 'QuartzCore'
 
+  # core files: UIGlossyButton.*. If you don't need layer effect, you can include
+  # the pod UIGlossyButton/Core only
   s.subspec 'Core' do |c|
   	c.source_files  = 'UIGlossyButton/UIGlossyButton.{h,m}'
   end
 
   # the layer effects file is for adding drop shadow effect to the button, actually not a must 
-  # for using UIGlossyButton, but you can include it using subspec, it is opt-out
+  # for using UIGlossyButton
   s.subspec 'LayerEffects' do |l|
   	l.source_files  = 'UIGlossyButton/UIView+LayerEffects.{h,m}'
   end