Update ConfigurationRoot.xml

Author: Grzegorz Suwaj

xml/Microsoft.Extensions.Configuration/ConfigurationRoot.xml

@@ -93,6 +93,8 @@
         <param name="providers">The <see cref="T:Microsoft.Extensions.Configuration.IConfigurationProvider" />s for this configuration.</param>
         <summary>Initializes a Configuration root with a list of providers.</summary>
         <remarks>To be added.</remarks>
+        <exception cref="T:System.ArgumentNullException">
+          <paramref name="providers" /> is <see langword="null" />.</exception>
       </Docs>
     </Member>
     <Member MemberName="Dispose">