Add guidance for package source

Author: pauljewellmsft

articles/storage/blobs/storage-quickstart-blobs-dotnet.md

@@ -85,6 +85,20 @@ To interact with Azure Blob Storage, install the Azure Blob Storage client libra
 dotnet add package Azure.Storage.Blobs
 ```
 
+If this command to add the package fails, see the following steps:
+
+- Make sure that `nuget.org` is added as a package source. You can list the package sources using the [dotnet nuget list source](/dotnet/core/tools/dotnet-nuget-list-source#examples) command:
+
+    ```dotnetcli
+    dotnet nuget list source
+    ```
+
+- If you don't see `nuget.org` in the list, you can add it using the [dotnet nuget add source](/dotnet/core/tools/dotnet-nuget-add-source#examples) command:
+
+    ```dotnetcli
+    dotnet nuget add source https://api.nuget.org/v3/index.json -n nuget.org
+    ```
+
 ---
 
 ### Set up the app code