Update readme to explain that setting ManagedIdentityClientId and … (Azure#18344)

Author: christothes

sdk/identity/Azure.Identity/README.md

@@ -107,6 +107,8 @@ var credential = new DefaultAzureCredential(new DefaultAzureCredentialOptions {
 var blobClient = new BlobClient(new Uri("https://myaccount.blob.core.windows.net/mycontainer/myblob"), credential);
 ```
 
+In addition to configuring the `ManagedIdentityClientId` via code, it can also be set using the `AZURE_CLIENT_ID` environment variable. These two approaches are equivalent when using the `DefaultAzureCredential`.
+
 ### Define a custom authentication flow with the `ChainedTokenCredential`
 While the `DefaultAzureCredential` is generally the quickest way to get started developing applications for Azure, more advanced users may want to customize the credentials considered when authenticating. The `ChainedTokenCredential` enables users to combine multiple credential instances to define a customized chain of credentials. This example demonstrates creating a `ChainedTokenCredential` which will attempt to authenticate using managed identity, and fall back to authenticating via the Azure CLI if managed identity is unavailable in the current environment. The credential is then used to authenticate an `EventHubProducerClient` from the [Azure.Messaging.EventHubs][eventhubs_client_library] client library.