[AzureMonitorExporter] add nightly to Readme (Azure#29671)

* add nightly to Readme

* remove extra space

* update with link to Dev Feed

Author: Timothy Mothra

sdk/monitor/Azure.Monitor.OpenTelemetry.Exporter/README.md

@@ -13,13 +13,18 @@ The [OpenTelemetry .NET](https://github.com/open-telemetry/opentelemetry-dotnet)
 
 ### Install the package
 
-Latest Version: [![Nuget](https://img.shields.io/nuget/vpre/Azure.Monitor.OpenTelemetry.Exporter.svg)](https://www.nuget.org/packages/Azure.Monitor.OpenTelemetry.Exporter/)  
+#### Latest Version: [![Nuget](https://img.shields.io/nuget/vpre/Azure.Monitor.OpenTelemetry.Exporter.svg)](https://www.nuget.org/packages/Azure.Monitor.OpenTelemetry.Exporter/)  
 
 Install the Azure Monitor Exporter for OpenTelemetry .NET with [NuGet](https://www.nuget.org/):
 ```dotnetcli
 dotnet add package Azure.Monitor.OpenTelemetry.Exporter --prerelease
 ```
 
+#### Nightly builds
+
+Nightly builds are available from this repo's [dev feed](https://github.com/Azure/azure-sdk-for-net/blob/main/CONTRIBUTING.md#nuget-package-dev-feed).
+These are provided without support and are not intended for production workloads.
+
 ### Authenticate the client
 
 Exporter does not use authentication.