Add ignore to umbraco folder (github#3307)

Add ignore to umbraco folder as it will be added by Install-Package UmbracoCms or updated by Update-Package UmbracoCms

Umbraco.gitignore

@@ -14,6 +14,11 @@
 # Ignore Umbraco content cache file
 **/App_Data/umbraco.config
 
+## this [Uu]mbraco/ folder should be created by cmd like `Install-Package UmbracoCms -Version 8.5.3` 
+## you can find your umbraco version at your Web.config. (i.e. <add key="Umbraco.Core.ConfigurationStatus" value="8.5.3" />)
+## Uncomment this line if you think it fits the way you work on your project.
+## **/[Uu]mbraco/ 
+
 # Don't ignore Umbraco packages (VisualStudio.gitignore mistakes this for a NuGet packages folder)
 # Make sure to include details from VisualStudio.gitignore BEFORE this
 !**/App_Data/[Pp]ackages/*