document Vim file patterns

Global/Vim.gitignore

@@ -1,6 +1,10 @@
+# swap
 [._]*.s[a-w][a-z]
 [._]s[a-w][a-z]
+# persistent undo 
 *.un~
+# session
 Session.vim
+# temporary
 .netrwhist
 *~