Add ignore for Secondary data files

Secondary data files .ndf (https://msdn.microsoft.com/en-us/library/ms189563.aspx)

VisualStudio.gitignore

@@ -219,6 +219,7 @@ UpgradeLog*.htm
 # SQL Server files
 *.mdf
 *.ldf
+*.ndf
 
 # Business Intelligence projects
 *.rdl.data
@@ -284,4 +285,4 @@ __pycache__/
 *.btp.cs
 *.btm.cs
 *.odx.cs
-*.xsd.cs
+*.xsd.cs