Add README.md for the Aspire.Hosting.Azure.AppService package (#12528)

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Co-authored-by: Shilpi Rachna <[email protected]>
Co-authored-by: Copilot <[email protected]>

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src/Aspire.Hosting.Azure.AppService/README.md

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+# Aspire.Hosting.Azure.AppService library
+
+Provides extension methods and resource definitions for an Aspire AppHost to configure Azure App Service for the compute resources (like project).
+
+## Getting started
+
+### Prerequisites
+
+- Azure subscription (requires Owner access to the target subscription for role assignments)
+
+### Install the package
+
+In your AppHost project, install the Aspire Azure App Service Hosting library with [NuGet](https://www.nuget.org):
+
+```dotnetcli
+dotnet add package Aspire.Hosting.Azure.AppService
+```
+
+## Usage example
+
+Then, in the _AppHost.cs_ file of `AppHost`, add an Azure App Service Environment and publish your project as an Azure App Service Web App:
+
+```csharp
+var builder = DistributedApplication.CreateBuilder(args);
+
+var appServiceEnvironment = builder.AddAzureAppServiceEnvironment("env");
+
+builder.AddProject<Projects.MyWebApp>("webapp")
+    .WithExternalHttpEndpoints()
+    .PublishAsAzureAppServiceWebsite((infrastructure, website) =>
+    {
+        // Customize the App Service health check path and appsettings
+        website.SiteConfig.HealthCheckPath = "/health";
+        website.SiteConfig.AppSettings.Add(new AppServiceNameValuePair()
+        {
+            Name = "Environment",
+            Value = "Production"
+        });
+    });
+```
+
+## Azure App Service constraints
+
+When deploying to Azure App Service, the following constraints apply:
+
+- **External endpoints only**: App Service only supports external endpoints. All endpoints must be configured using `WithExternalHttpEndpoints()`.
+- **HTTP/HTTPS only**: Only HTTP and HTTPS endpoints are supported. Other protocols are not supported.
+- **Single endpoint**: App Service supports only a single target port. Resources with multiple external endpoints with different target ports are not supported. The default target port is 8000, which can be overridden using the `WithHttpEndpoint` extension method.
+
+### Publishing compute resources to Azure App Service
+
+The `PublishAsAzureAppServiceWebsite` extension method is used to configure a compute resource (such as a project) to be published as an Azure App Service Web App when deploying to Azure. This method allows you to customize the App Service Web App configuration using the Azure Provisioning SDK.
+
+```csharp
+builder.AddProject<Projects.MyApi>("api")
+    .WithHttpEndpoint(targetPort: 8080)
+    .WithExternalHttpEndpoints()
+    .WithHealthProbe(ProbeType.Liveness, "/health")
+    .WithArgs("--environment", "Production")
+    .PublishAsAzureAppServiceWebsite((infrastructure, website) =>
+    {
+        // Customize the App Service Web App appsettings
+        website.SiteConfig.IsWebSocketsEnabled = true;
+        website.SiteConfig.MinTlsVersion = SupportedTlsVersions.Tls1_2;
+    });
+```
+
+### Adding an Azure App Service Environment
+
+The Azure App Service Environment resource creates the underlying infrastructure needed to host your applications, including:
+
+- An Azure App Service Plan (default SKU: P0v3)
+- An Azure Container Registry for storing container images
+- A managed identity for accessing the container registry
+- Optionally, the Aspire Dashboard as an Azure App Service Web App
+- Optionally, Application Insights for monitoring and telemetry
+
+```csharp
+var appServiceEnvironment = builder.AddAzureAppServiceEnvironment("env");
+```
+
+By default, the Aspire Dashboard is included in the App Service Environment. To disable the dashboard, use the `WithDashboard` extension method:
+
+```csharp
+var appServiceEnvironment = builder.AddAzureAppServiceEnvironment("env")
+    .WithDashboard(enable: false);
+```
+
+### Enabling Application Insights
+
+Application Insights can be enabled for the App Service Environment using the `WithAzureApplicationInsights` extension method. A different location can be specified for Application Insights using the optional location parameter:
+
+```csharp
+var appServiceEnvironment = builder.AddAzureAppServiceEnvironment("env")
+    .WithAzureApplicationInsights();
+```
+
+### Customizing the App Service Plan
+
+The App Service Plan can be customized using the `ConfigureInfrastructure` extension method.
+
+The default SKU for the App Service Plan is P0V3 and can be changed using this extension method:
+
+```csharp
+var appServiceEnvironment = builder.AddAzureAppServiceEnvironment("env")
+    .ConfigureInfrastructure((infra) =>
+    {
+        var plan = infra.GetProvisionableResources().OfType<AppServicePlan>().Single();
+        plan.Sku = new AppServiceSkuDescription
+        {
+            Name = "P2V3",
+            Tier = "Premium"
+        };
+    });
+```
+
+## Additional documentation
+
+* https://learn.microsoft.com/azure/app-service/
+
+## Feedback & contributing
+
+https://github.com/dotnet/aspire