Add some instruction on how to build it

[Kongolan]

I don't know what file to build for the exe and how to build the source anyways. So please add some short instructions, so i can develop it further.

• Which python version to use
• If you used pyinstaller
• What command to use
• Which file the main file is, perhaps it's randomizer_gui.py?

Just add something to the readme like this: To build it from source use the following command with python 3.8 and pyinstaller.