Created Linux usage instructions (markdown)

Linux-usage-instructions.md

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+Credits to [@RenanYudi](https://github.com/RenanYudi)
+
+You can use DiscordChatExporter with CLI and Mono. It isn't that user friendly, but it works.
+
+---
+**Installing Mono**
+To get mono on Ubuntu/Debian ([other distros and more info here](https://www.mono-project.com/download/stable/#download-lin-ubuntu)): 
+Open Terminal and type
+1. `sudo apt-get install mono-devel`
+
+You can check if mono was installed successfully using `mono -V`, it should output its version. If it didn't work, try the link above for more info.
+
+---
+
+**Downloading and using DiscordChatExporter**
+
+1. Download the latest DiscordChatExporter CLI release [here](https://github.com/Tyrrrz/DiscordChatExporter/releases/latest) and extract its contents on your Desktop to a folder called `DiscordChatExporter`.
+
+2. Open terminal and type to change Terminal directory:
+`cd ~/Desktop/DiscordChatExporter`
+
+3. Replace `TOKEN` and `CHANNEL`, and execute the command to run DiscordChatExporter.Cli.exe:
+`mono DiscordChatExporter.Cli.exe -t TOKEN -c CHANNEL`
+---
+It's also possible to run the CLI on macOS, but installing mono is a little bit different, since you'd be using brew or macports instead of apt.
+
+The GUI version doesn't work on non-Windows OS.