Document mingw and mswin in the README

Author: eregon

README.md

@@ -17,11 +17,12 @@ This action currently supports these versions of MRI, JRuby and TruffleRuby:
 
 | Interpreter | Versions |
 | ----------- | -------- |
-| Ruby | 2.2, 2.3.0 - 2.3.8, 2.4.0 - 2.4.9, 2.5.0 - 2.5.7, 2.6.0 - 2.6.5, 2.7.0, head |
+| Ruby | 2.2, 2.3.0 - 2.3.8, 2.4.0 - 2.4.9, 2.5.0 - 2.5.7, 2.6.0 - 2.6.5, 2.7.0, head, mingw, mswin |
 | JRuby | 9.2.9.0 - 9.2.10.0, head |
 | TruffleRuby | 19.3.0 - 20.0.0, head |
 | Rubinius | 4.14 |
 
+`mingw` and `mswin` are `ruby-head` builds using the MSYS2 and MSVC toolchains on Windows.  
 Note that Ruby ≤ 2.3 and the OpenSSL version it needs (1.0.2) are both end-of-life,
 which means Ruby ≤ 2.3 is unmaintained and considered insecure.
 On Windows, Ruby 2.4 uses OpenSSL 1.0.2, which is no longer maintained.
@@ -41,6 +42,7 @@ Rubinius is only available on `ubuntu-latest`.
 
 The prebuilt releases are generated by [ruby-builder](https://github.com/ruby/ruby-builder)
 and on Windows by [RubyInstaller2](https://github.com/oneclick/rubyinstaller2).
+`mingw` 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)
 and `truffleruby-head` is generated by [truffleruby-dev-builder](https://github.com/ruby/truffleruby-dev-builder).