Add wiki to repository

[Seeloewen]

Right now, information on the Random Item Giver and frequently asked questions are scattered across many platforms. Some things are on this repository, some are on Discord, others are on Planetminecraft. Finally have it all in one place and for better maintainability, I have decided to create a wiki on this site which will contain everything you need to know about the Random Item Giver.

Things it will contain:

• Guide on how to install on singleplayer/multiplayer
• Common issues and solutions
• Frequently asked questions
• Guide on how to add custom items
• Supported versions and where to download
• A little bit of backstory with credits to Phoenix SC
• Which things can be accessed with op-only?

Of course, the wiki is not limited to those things. It will evolve over time and if you feel like I forgot something, don't hesitate to open a pull request or an issue!