ignore Visual Studio 6 workspace file

see e.g.
http://stackoverflow.com/a/1530193
https://msdn.microsoft.com/en-us/library/aa241721(v=vs.60).aspx

VisualStudio.gitignore

@@ -239,6 +239,9 @@ FakesAssemblies/
 # Visual Studio 6 workspace options file
 *.opt
 
+# Visual Studio 6 auto-generated workspace file (contains which files were open etc.)
+*.vbw
+
 # Visual Studio LightSwitch build output
 **/*.HTMLClient/GeneratedArtifacts
 **/*.DesktopClient/GeneratedArtifacts