Improve doc about Telegram notifier

[TBG-FR]

README.md is missing information about Telegram notifier/bot configuration

Instructions (to improve) :

• Following instructions given on #7
• Adding env variable TELEGRAM_TOKEN (token given by @Botfather)
• Getting chat id by sending /start to the bot
• Adding a new telegram notifier with chat id as username and a random value as password

[Eywek]

Hello,

You can contribute via PRs to add this kind of documentation that could be really helpful

[TBG-FR]

I'll try to as soon as I can, on documentation and code 😄

[yann120]

Thanks, I added the feature but no documentation, it's true @TBG-FR Add me in review once you have a draft :)