Add NuGet packages folder to .gitignore for convenience
Add NuGet packages folder to .gitignore for convenience

vbjay/ClosedXML,clinchergt/ClosedXML,jongleur1983/ClosedXML,JavierJJJ/ClosedXML,igitur/ClosedXML,b0bi79/ClosedXML,ClosedXML/ClosedXML