[TargetHeader] Add warning.

lib/cocoapods/generator/target_header.rb

@@ -45,6 +45,7 @@ module Pod
       #
       def save_as(pathname)
         pathname.open('w') do |source|
+          source.puts "// WARNING: This feature of CocoaPods is present for discussion purposes and might be discontinued or changed in future"
           source.puts "#define __COCOA_PODS"
           source.puts
           specs.each do |specs|

spec/unit/generator/target_header_spec.rb

@@ -11,6 +11,7 @@ describe Pod::Generator::TargetHeader do
     file = temporary_directory + 'PodsDummy.m'
     @gen.save_as(file)
     file.read.should == <<-EOS.strip_heredoc
+    // WARNING: This feature of CocoaPods is present for discussion purposes and might be discontinued or changed in future
     #define __COCOA_PODS
 
     #define __POD_BananaLib