Add readme for Ambient Metadata

src/Libraries/Microsoft.Extensions.AmbientMetadata.Application/README.md

@@ -1 +1,67 @@
-README
+# Microsoft.Extensions.AmbientMetadata.Application
+
+This flows runtime information for application-level ambient metadata such as the version, deployment ring, environment, and name. This information can be useful to enrich telemetry.
+
+## Install the package
+
+From the command-line:
+
+```dotnetcli
+dotnet add package Microsoft.Extensions.AmbientMetadata.Application
+```
+
+Or directly in the C# project file:
+
+```xml
+<ItemGroup>
+  <PackageReference Include="Microsoft.Extensions.AmbientMetadata.Application" Version="[CURRENTVERSION]" />
+</ItemGroup>
+```
+
+## Usage Example
+
+### Registering Services
+
+The services can be registered using any of the following methods:
+
+```csharp
+public static IHostBuilder UseApplicationMetadata(this IHostBuilder builder, string sectionName = DefaultSectionName)
+public static IServiceCollection AddApplicationMetadata(this IServiceCollection services, Action<ApplicationMetadata> configure)
+```
+
+### Configuration
+
+When loading from configuration, the version and deployment ring metadata are read from the `ambientmetadata:application` section. The environment and application names are read from the `IHostEnvironment`.
+
+```json
+{
+  "AmbientMetadata" {
+    "Application" {
+      "BuildVersion": "1.0-alpha1.2346",
+      "DeploymentRing": "InnerRing"
+    }
+  }
+}
+```
+
+### Consuming Services
+
+The `ApplicationMetadata` can be injected wherever needed. For example:
+
+```csharp
+public class MyClass
+{
+  public MyClass(IOptions<ApplicationMetadata> options) { Application = options.Value; }
+
+  private ApplicationMetadata Application { get; }
+
+  public void DoWork()
+  {
+    Log.LogEnvironment(Application.Version, Application.DeploymentRing, Application.Environment, Application.Name);
+  }
+}
+```
+
+## Feedback & Contributing
+
+We welcome feedback and contributions in [our GitHub repo](https://github.com/dotnet/extensions).