[DOC] Cross-link between README pages

Author: BurdetteLamar

README.ja.md

@@ -4,6 +4,8 @@
 [![AppVeyor status](https://ci.appveyor.com/api/projects/status/0sy8rrxut4o0k960/branch/master?svg=true)](https://ci.appveyor.com/project/ruby/ruby/branch/master)
 [![Travis Status](https://app.travis-ci.com/ruby/ruby.svg?branch=master)](https://app.travis-ci.com/ruby/ruby)
 
+[English](rdoc-ref:README.md)
+
 # Rubyとは
 
 Rubyはシンプルかつ強力なオブジェクト指向スクリプト言語です． Rubyは純粋なオブジェクト指向言語として設計されているので，

README.md

@@ -3,6 +3,8 @@
 [![Actions Status: Windows](https://github.com/ruby/ruby/workflows/Windows/badge.svg)](https://github.com/ruby/ruby/actions?query=workflow%3A"Windows")
 [![Travis Status](https://app.travis-ci.com/ruby/ruby.svg?branch=master)](https://app.travis-ci.com/ruby/ruby)
 
+[日本語](rdoc-ref:README.ja.md)
+
 # What is Ruby?
 
 Ruby is an interpreted object-oriented programming language often