Added documentation for demo module.

Closes #40
Closes #61

demo/README.md

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+The `demo` module is a **special module** that provides a **fully functioning Spigot plugin**.
+The main purpose of this module is to provide an actual **test** of **all YAGL features** for the user.
+
+`demo` is compatible for **every Minecraft version** listed in the main page, 
+and uses a special programming language for all its commands: **Groovy**.
+
+With it, `demo` gives capability to the end user to **view and modify** the code ran upon executing demo commands
+**without recompiling the project**, by simply **reloading the plugin**.
+
+All the available commands will be listed in the [resources directory](src/main/resources/commands).