Add a sentence about the new signature of Guard::Plugin#initialize

Upgrading-to-Guard-2.0.md

@@ -4,13 +4,18 @@ _This is a WIP, please not edit. Thanks, [@rymai](https://github.com/rymai)._
 
 ### `Guard::Guard` deprecated in favor of `Guard::Plugin`
 
-To remove the confusion between Guard and its many plugins (guard-rspec, guard-pow, guard-livereload etc.), `Guard::Guard` is renamed `Guard::Plugin`. `Guard::Guard` is deprecated so if you're a plugin maintainer, in your next major release you should make your plugin inherit from `Guard::Plugin` instead of `Guard::Guard`.
+To remove the confusion between Guard and its many plugins (guard-rspec, guard-pow, guard-livereload etc.), `Guard::Guard` is renamed `Guard::Plugin`. `Guard::Guard` is deprecated so if you're a plugin maintainer, in your next (major) release **that will depend on Guard ~> 2.0** you must make your plugin inherit from `Guard::Plugin` instead of `Guard::Guard`.
 
-For instance, for guard-bundler:
+Also, please be sure to change the `#initialize` signature from `#initialize(watchers = [], options = {})` to `#initialize(options = {})` (watchers are now passed in the `options` argument directly).
+
+For instance, for guard-rspec:
 ```ruby
 module Guard
-  class Bundler < Guard
-    # implementation
+  class RSpec < Guard
+    def initialize(watchers = [], options = {})
+      super
+      # rest of the implementation...
+    end
   end
 end
 ```
@@ -20,7 +25,10 @@ should become:
 ```ruby
 module Guard
   class Bundler < Plugin
-    # implementation
+    def initialize(options = {})
+      super
+      # rest of the implementation...
+    end
   end
 end
-```
+```