1. Create a new Telegram bot:
   1. start a chat with BotFather;
   2. type /newbot, select a name (to be shown in chats) and handle for your bot;
   3. note the bot token to access HTTP API - a long string with a colon in the middle (later referred to as <token>);
   4. optionally /setdescription and /setuserpic.
2. Add the bot to the chat/channel and note its id:
   1. add the bot to a new or existing chat or group - this is where your bot will send notifications to;
   2. go to https://api.telegram.org/bot<token>/getUpdates (replace <token> with your token);
   3. within the chat part, find and note the id field - a positive or negative number (later referred to as <chat_id>).
3. Programmatically send notifications:
   1. Essentially, you just need to construct a special URL and open it;
   2. For example, this can be done like this in Python (replace <token>, <chat_id> and <message>):

      import urllib, requests
      url = 'https://api.telegram.org/bot%s/sendMessage?chat_id=%s&text=%s' % (
          '<token>', '<chat_id>', urllib.parse.quote_plus('<message>'))
      _ = requests.get(url, timeout=10)

      Depending on your version of python, urllib.parse.quote_plus could be urllib.quote_plus.
   3. For options, refer to the official documentation.

To use telegram notifications with supervisor, modify /etc/supervisor/supervisord.conf to include the following lines (possible events):

[eventlistener:telegramnotitfactor]
command=curl "https://api.telegram.org/bot<token>/sendMessage?chat_id=<chat_id>&text=<message>"
events=PROCESS_STATE_STARTING