Appveyor: Explain appveyor image names in the comments

Appveyor images are named after the Visual Studio version they contain.
But we compile using MinGW, not Visual Studio.
We use these images because they have different Windows versions.

Closes bug 28826.

.appveyor.yml

@@ -2,6 +2,9 @@ version: 1.0.{build}
 
 clone_depth: 50
 
+# Appveyor images are named after the Visual Studio version they contain.
+# But we compile using MinGW, not Visual Studio.
+# We use these images because they have different Windows versions.
 image:
   # Windows Server 2012 R2
   - Visual Studio 2015