Added MFractor's working folder to VisualStudio.gitignore

MFractor is a Xamarin/Visual Studio Mac productivity tool used by 1000's of Xamarin developers.

The `.mfractor/` folder should not be included in source control (but often is).

VisualStudio.gitignore

@@ -324,3 +324,6 @@ ASALocalRun/
 
 # NVidia Nsight GPU debugger configuration file
 *.nvuser
+
+# MFractors (Xamarin productivity tool) working folder 
+.mfractor/