Make it clear not all combinations are supported

Author: eregon

README.md

@@ -53,15 +53,17 @@ The action works on these [GitHub-hosted runners](https://docs.github.com/en/act
 | macOS   | `macos-13` and newer versions |
 | Windows | `windows-2019`, `windows-2022` |
 
+Not all combinations of runner image and versions are supported.
+The list of available Ruby versions per runner image can be seen in [ruby-builder-versions.json](ruby-builder-versions.json) for Ubuntu and macOS
+(although some combinations are not available, see [the full list](https://github.com/ruby/ruby-builder/releases/tag/toolcache))
+and in [windows-versions.json](windows-versions.json) for Windows.
+
 The prebuilt releases are generated by [ruby-builder](https://github.com/ruby/ruby-builder)
 and on Windows by [RubyInstaller2](https://github.com/oneclick/rubyinstaller2).
 The `mingw`, `ucrt` and `mswin` builds are generated by [ruby-loco](https://github.com/MSP-Greg/ruby-loco).
 `ruby-head` is generated by [ruby-dev-builder](https://github.com/ruby/ruby-dev-builder),
 `jruby-head` is generated by [jruby-dev-builder](https://github.com/ruby/jruby-dev-builder),
 `truffleruby-head` and `truffleruby+graalvm-head` are generated by [truffleruby-dev-builder](https://github.com/ruby/truffleruby-dev-builder).
-The full list of available Ruby versions can be seen in [ruby-builder-versions.json](ruby-builder-versions.json) for Ubuntu and macOS
-(although some combinations are not available, see [the full list](https://github.com/ruby/ruby-builder/releases/tag/toolcache))
-and in [windows-versions.json](windows-versions.json) for Windows.
 
 ## Usage