docs: clarify INSTALL.md regarding CHANNEL property in .env file (#549)

docs/INSTALL.md

@@ -157,7 +157,7 @@ Details of some variables:
 
 - _BOT_TOKEN:_ You will need to go to Telegram and open [@BotFather](https://t.me/BotFather). Run the menu and choose `Create a new bot` where you will select the bot's name and username. Once generated, it will show you a token that you will place in this field.
 
-- _CHANNEL:_ This is the channel where the bot posts offers. Create a channel on Telegram by pressing the button to write a new message. On Android, it is in the bottom right corner with a round icon featuring a pencil, and on iOS, it is in the top right corner with a small pencil-shaped icon. Press the `New Channel` option. Then, set the bot as a channel's administrator (open the bot and select: _Add to a group or channel_).
+- _CHANNEL:_ This is the channel where the bot posts offers. Create a channel on Telegram by pressing the button to write a new message. On Android, it is in the bottom right corner with a round icon featuring a pencil, and on iOS, it is in the top right corner with a small pencil-shaped icon. Press the `New Channel` option. Then, set the bot as a channel's administrator (open the bot and select: _Add to a group or channel_). Channel has to be public. Part after `t.me/` in link shown in channel info is what goes into __CHANNEL__ property (e.g. if link is `t.me/foo` then value should be `@foo`).
 
 - _ADMIN_CHANNEL:_ This channel is where the bot sends relevant information to administrators, such as payments failures, when a user takes an order but doesn't proceed, among other things. In this field, you need to enter the channel ID. To obtain it, write a message in the channel, forward it to the `@JsonDumpBot`, and it will show you a JSON with the channel ID. More information can be found [here](https://gist.github.com/mraaroncruz/e76d19f7d61d59419002db54030ebe35). You need to add the bot as a channel's administrator.