added simple AsyncIO example. (#188)

* added simple AsyncIO example.

* added gif to README.md file.

* Update main.py to satisfy linter

---------

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

Author: stephenlb

examples/pubnub_asyncio_simple/README.md

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+# AsyncIO PubNub Subscribe Example
+
+![pubnub-asyncio-simple-example](https://gist.github.com/assets/45214/07223c2e-a5f0-453d-91b2-819fcb526ab5)
+
+### Usage example:
+```shell
+pip install asyncio pubnub
+export PUBNUB_PUBLISH_KEY=demo
+export PUBNUB_SUBSCRIBE_KEY=demo
+python main.py
+```
+
+### Output:
+```
+Listening for messages...
+Connected
+Received message: Hello World on channel: my_channel
+Received message: Hello World on channel: my_channel
+Received message: Hello World on channel: my_channel
+Received message: Hello World on channel: my_channel
+Received message: Hello World on channel: my_channel
+```
+
+
+### In another terminal:
+```shell
+export PUBNUB_PUBLISH_KEY=demo
+export PUBNUB_SUBSCRIBE_KEY=demo
+curl "https://ps.pndsn.com/publish/${PUBNUB_PUBLISH_KEY}/${PUBNUB_SUBSCRIBE_KEY}/0/my_channel/0/%22Hello%20World%22"
+```
+
+### Output:
+```
+[1,"Sent","17183967137027574"]
+```

examples/pubnub_asyncio_simple/main.py

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+import os
+import asyncio
+
+from pubnub.pnconfiguration import PNConfiguration
+from pubnub.pubnub_asyncio import PubNubAsyncio, SubscribeCallback
+from pubnub.enums import PNStatusCategory
+
+
+class MySubscribeCallback(SubscribeCallback):
+    def status(self, pubnub, status):
+        if status.category == PNStatusCategory.PNUnexpectedDisconnectCategory:
+            print("Disconnected")
+        elif status.category == PNStatusCategory.PNConnectedCategory:
+            print("Connected")
+        elif status.category == PNStatusCategory.PNReconnectedCategory:
+            print("Reconnected")
+        elif status.category == PNStatusCategory.PNDecryptionErrorCategory:
+            print("Decryption error")
+
+    def message(self, pubnub, message):
+        print(f"Received message: {message.message} on channel: {message.channel}")
+
+    def presence(self, pubnub, presence):
+        print(f"Presence event: {presence.event}")
+
+
+async def main(pubnub):
+    pubnub.subscribe().channels('my_channel').execute()
+    print("Listening for messages...")
+    while True:
+        await asyncio.sleep(1)
+
+if __name__ == "__main__":
+    loop = asyncio.new_event_loop()
+    asyncio.set_event_loop(loop)
+
+    pnconfig = PNConfiguration()
+    pnconfig.subscribe_key = os.getenv('PUBNUB_SUBSCRIBE_KEY') or 'demo'
+    pnconfig.publish_key = os.getenv('PUBNUB_PUBLISH_KEY') or 'demo'
+    pnconfig.user_id = "my_unique_user_id"  # Set a unique user ID
+
+    pubnub = PubNubAsyncio(pnconfig)
+    callback = MySubscribeCallback()
+    pubnub.add_listener(callback)
+
+    try:
+        loop.run_until_complete(main(pubnub))
+    except KeyboardInterrupt:
+        print("Interrupted by user. Exiting...")
+    finally:
+        loop.run_until_complete(pubnub.stop())  # Assuming 'pubnub' is in scope
+        loop.close()