doc: Document alternative to set Gov flag (#1621)

Author: lantoli

README.md

@@ -104,6 +104,14 @@ SecretName: cfn/atlas/profile/{ProfileName}
 SecretValue: {"PublicKey": "YourPublicKey", "PrivateKey": "YourPrivateKey", "IsMongoDBGovCloud": true}
 ```
 
+Using `IsMongoDBGovCloud` is the recommended approach. Alternatively, you can set the `BaseUrl` field directly to the Gov Cloud URL. The following configuration is equivalent:
+
+```
+SecretValue: {"PublicKey": "YourPublicKey", "PrivateKey": "YourPrivateKey", "BaseUrl": "https://cloud.mongodbgov.com/"}
+```
+
+If both `BaseUrl` and `IsMongoDBGovCloud` are set, `BaseUrl` takes precedence.
+
 ### Prerequisites
 1. Review [Atlas for Government considerations](https://www.mongodb.com/docs/atlas/government/atlas-for-government/).
 2. An existing Atlas Organization with billing set up in your Cloud Gov environment.