Ensure no warnings in YARD

Resolves #22

.yardopts

@@ -0,0 +1 @@
+--fail-on-warning

Rakefile

@@ -3,7 +3,7 @@ require 'bundler/gem_tasks'
 
 require 'rake/testtask'
 
-default_tasks = [:test]
+default_tasks = [:test, :test_yard]
 if Gem::Version.create(RUBY_VERSION) >= Gem::Version.create('2.6.0')
   default_tasks << :'signature:validate'
 end
@@ -20,3 +20,7 @@ namespace :signature do
     sh 'bundle exec rbs -rsingleton -I sig validate'
   end
 end
+
+task :test_yard do
+  sh "bundle exec yard --fail-on-warning #{'--no-progress' if ENV['CI']}"
+end