add documentation for access groups

README.md

@@ -20,7 +20,8 @@ Trifle SDK is implemented on client side in iOS and Android and on server side i
 
 ```swift
 // App start up
-let trifle = try Trifle(reverseDomain: abc)
+// The access group value must be added to the App Groups [entitlement file](https://developer.apple.com/documentation/bundleresources/entitlements/com_apple_security_application-groups).
+let trifle = try Trifle(reverseDomain: abc, accessGroup: "group.trifle.cash.app")
 
 // Check if a key already exists.
 // If no key exists, generate a public key pair

ios/Trifle/Sources/Trifle.swift

@@ -30,6 +30,7 @@ public class Trifle {
      attribute will mean that all key related APIs will be limited to the specified access group
      (of which the calling application must be a member to obtain matching results.)
      It is recommended that this value is set.
+    ** This value must be added to the App Group entitlement file. **
 
      If the access group is not set, Trifle keys are created in the application's default access group.
      */