readme: Remove the note about only being tested on macos and linux

It has recently been tested on msys2 as well, with few changes since
the last time it was tested.

README.md

@@ -26,8 +26,7 @@ for windows come in 4 versions, one for each of the 4 supported
 architectures, but each one of them can target all 4 architectures.
 
 The toolchain can be reproducibly built into a Docker image, or be
-built and installed in the host environment (currently only tested
-on Linux and macOS).
+built and installed in the host environment.
 
 To build and install all components, just do: