Try to fix some broken links

see https://github.com/ruby/docs.ruby-lang.org/issues/56

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63131 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

.document

@@ -24,4 +24,8 @@ NEWS
 README.md
 README.ja.md
 
+COPYING
+COPYING.ja
+CONTRIBUTING.md
+
 doc