[README] Add GitPOAP Badge to Display Number of Minted GitPOAPs for Contributors

[nixorokish]

Hey all, this PR adds a GitPOAP Badge to the README that displays the number of minted GitPOAPs for this repository by contributors to this repo.

You can see an example of this in our Documentation repository.

This should help would-be contributors as well as existing contributors find out that they will/have received GitPOAPs for their contributions.

CC: @colfax23 @kayla-henrie