Update NU1100.md (#2829)

NU1100 may also be caused by the lack of a NuGet configura

docs/reference/errors-and-warnings/NU1100.md

@@ -17,11 +17,16 @@ f1_keywords:
 <pre>Unable to resolve 'Dependency (>= 1.0.0)' for 'TargetFramework'</pre>
 
 ### Issue
-A dependency could not be resolved. This is a generic issue for types that are not packages or projects.
+A dependency could not be resolved. This is a generic issue for types that are not packages or projects. 
+
+This issue may also be caused by the lack of a NuGet configuration file.
 
 ### Solution
 Open the project file and examine the list of its dependencies. Check that each dependency exists on the package sources you're using, and that the package supports the project's target framework.
 
+In the case of a NuGet configuration file not being created, issue the following command from the root folder of the project:  
+`dotnet new nugetconfig`
+
 ## Scenario 2
 
 <pre> Unable to resolve 'Dependency (>= 1.0.0)' for 'TargetFramework'.  PackageSourceMapping is enabled, the following source(s) were not considered: nuget. </pre>