spec/README.md: add a note about version guards

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64410 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/README.md

@@ -19,6 +19,9 @@ In other words: If adding a spec might reveal a bug in
 another implementation, then it is worth adding it.
 Currently, the only module which is MRI-specific is `RubyVM`.
 
+Version guards (`ruby_version_is`) must be added for new features or features
+which change behavior or are removed. See `spec/ruby/CONTRIBUTING.md` for details.
+
 ## Running ruby/spec
 
 To run all specs: