Adding MobX as an Alternative State Manager Example

[codeBelt]

Jack Herrington has a video titled STOP Using Recoil, Use This Instead!, where he invites the community to suggest their preferred state manager. I decided to add MobX! I followed the format of the other examples, and I hope I did justice to MobX within those constraints.

Examples are found here: https://github.com/jherr/recoil-alternatives

[kubk]

@codeBelt Looks great, thank you! 👍