Added section for SQL Server files

Ignore SQL Server Database and log files in App_Data.

CSharp.gitignore

@@ -106,3 +106,7 @@ Generated_Code #added for RIA/Silverlight projects
 _UpgradeReport_Files/
 Backup*/
 UpgradeLog*.XML
+
+# SQL Server files
+App_Data/*.mdf
+App_Data/*.ldf