Add clarification notes on configuration parameter usage

In relation to #47, I just wanted to contribute a few bits of documentation which might help users figure out whay are they not getting Telegram notifications. The way to determine a chat group id comes from [this StackOverflow answer](https://stackoverflow.com/questions/32423837/telegram-bot-how-to-get-a-group-chat-id#32572159).

Hope this helps. Thanks.

# `PLUGIN_TOKEN` and `PLUGIN_TO` parameter configuration.

- `PLUGIN_TOKEN` is the token issued by [@BotFather](https://telegram.me/botfather) when configuring a new bot, as described in [Telegram's Bot API](https://core.telegram.org/bots#3-how-do-i-create-a-bot).

- `PLUGIN_TO` consist of a numerical id, which specifies the Telegram chat group where the Telegram plugin will send build notifications. To obtain the chat group id:

- Call the telegram API using your bot's token:

https://api.telegram.org/bot/getUpdates

And extract the `id` value from the `chat` object in the JSON response.

{
"ok": true,
"result": [{
"update_id": 549000058,
"message": {
"message_id": 3,
"from": {
"id": 000000000,
"is_bot": false,
"first_name": "AHumansFirstName",
"last_name": "AHumansLastName",
"username": "humanUsername",
"language_code": "en"
},
"chat": {
"id": 123456789,
"first_name": "AHumansFirstName",
"last_name": "AHumansLastName",
"username": "humanUsername",
"type": "private"
},
"date": 1513466390,
"text": "LOLZ"
}
}]
}

No target documentation file, test, or entry point is named. Start by locating the existing configuration documentation for PLUGIN_TOKEN and PLUGIN_TO, then add the BotFather and getUpdates guidance from the issue and verify that the Markdown renders correctly.

Written by the indexing model from the issue text.