Added reference to get latest from GitHub.

If the .gitignore file is inlcuded in another project, it may be feasible to have a reference to origin.

VisualStudio.gitignore

@@ -1,5 +1,7 @@
 ## Ignore Visual Studio temporary files, build results, and
 ## files generated by popular Visual Studio add-ons.
+##
+## Get latest from https://github.com/github/gitignore/blob/master/VisualStudio.gitignore
 
 # User-specific files
 *.suo