Clarify about mingw and mswin

From #24 (comment)

Author: eregon

README.md

@@ -22,7 +22,7 @@ This action currently supports these versions of MRI, JRuby and TruffleRuby:
 | 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.  
+On Windows, `mingw` and `mswin` are `ruby-head` builds using the MSYS2/MinGW and the MSVC toolchains respectively.  
 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 On Windows, Ruby 2.4 uses OpenSSL 1.0.2, which is no longer maintained.