Add info about the -head builders

Author: eregon

README.md

@@ -32,8 +32,9 @@ The action works for all [GitHub-hosted runners](https://help.github.com/en/acti
 
 Rubinius is only available on `ubuntu-latest`.
 
-The prebuilt rubies are generated by [eregon/ruby-install-builder](https://github.com/eregon/ruby-install-builder)
-and on Windows by [RubyInstaller2](https://github.com/oneclick/rubyinstaller2).  
+The prebuilt releases are generated by [ruby-install-builder](https://github.com/eregon/ruby-install-builder)
+and on Windows by [RubyInstaller2](https://github.com/oneclick/rubyinstaller2).
+`ruby-head` is generated by [ruby-dev-builder](https://github.com/eregon/ruby-dev-builder) and `truffleruby-head` is generated by [truffleruby-dev-builder](https://github.com/eregon/truffleruby-dev-builder).
 The full list of available Ruby versions can be seen in [ruby-install-builder-versions.js](ruby-install-builder-versions.js)
 for Ubuntu and macOS and in [windows-versions.js](windows-versions.js) for Windows.