updated CHANGES.md and README.md

Author: maxkahan

CHANGES.md

@@ -2,6 +2,21 @@
 - Moved some client methods into their own classes: `account.py, application.py, 
 message_search.py, number_insight.py, numbers.py, short_codes.py, ussd.py`
 - Deprecated the corresponding client methods. These will be removed in a major release that's coming soon.
+- Client now instantiates a class object for each API when it is created, e.g. `vonage.Client(key="mykey", secret="mysecret")`
+instantiates instances of `Account`, `Sms`, `NumberInsight` etc. These instances can now be called directly from `Client`, e.g.
+```
+client = vonage.Client(key="mykey", secret="mysecret")
+
+print(f"Account balance is: {client.account.get_balance()}")
+
+print("Sending an SMS")
+client.sms.send_message(
+    "from": "Vonage",
+    "to": "SOME_PHONE_NUMBER",
+    "text": "Hello from Vonage's SMS API"
+)
+
+```
 
 # 2.6.x