add rule to keep track of directories ending in .Cache

More Information
http://blogs.msdn.com/b/msbuild/archive/2008/02/11/what-s-up-with-xxx-sln-cache.aspx

Author: LunicLynx

VisualStudio.gitignore

@@ -150,8 +150,13 @@ csx/
 # Windows Store app package directory
 AppPackages/
 
-# Others
+# Visual Studio cache files
+# files endeing in .cache can be ignored
 *.[Cc]ache
+# but keep track of directories ending in .cache
+!*.[Cc]ache/
+
+# Others
 ClientBin/
 [Ss]tyle[Cc]op.*
 ~$*