Update README

README.md

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 # AWS AppConfig Configuration Provider for .NET
 
-TODO
+An opinionated [.NET Configuration Provider](https://learn.microsoft.com/en-us/dotnet/core/extensions/configuration-providers)
+for AWS AppConfig that wraps [Amazon.Extensions.Configuration.SystemsManager](https://www.nuget.org/packages/Amazon.Extensions.Configuration.SystemsManager/).
 
+## Usage
+
+First, download the provider from NuGet:
+
+```shell
+dotnet add package CatConsult.AppConfigConfigurationProvider
+```
+
+Then add the provider using the `AddAppConfig` extension method:
+
+```csharp
+builder.Configuration.AddAppConfig();
+```
+
+By default, the provider will look for a configuration section named `AppConfig`.
+This can be overriden by specifying a different section name:
+
+```csharp
+builder.Configuration.AddAppConfig("MyCustomName");
+```
+
+## Configuration
+
+The provider requires some minimal configuration in order for it to know which AppConfig profiles to load: 
+
+```json
+{
+  "AppConfig": {
+    "Profiles": [
+      "abc1234:def5678:ghi9123",
+      "q2w3e25:po92j45:bt9s090:300"
+    ]
+  }
+}
+```
+
+As you can see in the example above, the AppConfig metadata are encoded as a formatted string:
+
+```
+ApplicationId:EnvironmentId:ProfileId[:ReloadAfter]
+```
+
+`ReloadAfter` is an optional 4th parameter that configures the reload/refresh period.
+It is an integer that represents time in seconds.
+If not specified, it defaults to the provider's default setting, which is 30 seconds.
+
+The default `ReloadAfter` setting can be overridden as well:
+
+```json
+{
+  "AppConfig": {
+    "Defaults": {
+      "ReloadAfter": 90
+    }
+  }
+}
+```