Add experimental warning

lib/rbs/cli.rb

@@ -673,6 +673,7 @@ def run_prototype(args, options)
             merge = true
           end
           opts.on("--todo", "Generates only undefined methods compared to objects") do
+            Warning.warn("Geneating prototypes with `--todo` option is experimental\n", category: :experimental)
             todo = true
           end
           opts.on("--method-owner CLASS", "Generate method prototypes if the owner of the method is [CLASS]") do |klass|

test/rbs/cli_test.rb

@@ -681,6 +681,30 @@ module A
     end
   end
 
+  def test_prototype__runtime__todo
+    Dir.mktmpdir do |dir|
+      Dir.chdir(dir) do
+        with_cli do |cli|
+          begin
+            old = $stderr
+            $stderr = cli.stderr
+            cli.run(%w(prototype runtime --todo ::Object))
+          ensure
+            $stderr = old
+          end
+
+          assert_equal <<~EOM, cli.stdout.string
+          EOM
+
+          assert_equal <<~EOM, cli.stderr.string
+            Geneating prototypes with `--todo` option is experimental
+          EOM
+        end
+      end
+    end
+  end
+
+
   def test_test
     Dir.mktmpdir do |dir|
       dir = Pathname(dir)