Add a section for MonoGame milestones/progress

[SimonDarksideJ]

Link to Document

https://github.com/MonoGame/MonoGame/milestone/10

Description

Ideally, there should be a page to showcase the Milestones / progress for MonoGame, to provide better transparency for the MonoGame roadmap
https://github.com/MonoGame/MonoGame/milestone/10