docs: readme usage update (#82)

Co-authored-by: [email protected] <[email protected]>

Author: kierramarie

README.md

@@ -51,6 +51,12 @@ https://terraform-ibm-modules.github.io/documentation/#/implementation-guideline
 ### Usage
 
 ```hcl
+# Provider aliases
+providers = {
+    ibm              = ibm
+    ibm.secret-store = ibm
+}
+
 # Authentication with IAM policy
 module "public_secret_engine" {
   source                       = "terraform-ibm-modules/secrets-manager-public-cert-engine/ibm"