Fix showcase submissions

[tomspilman]

So the current showcase submission issue template has a few issues:

• Our old submissions use 1280x720 screenshots... we are asking for 1920x1080. Are we worried about (the viewers) bandwidth loading dozens of these screenshots at once.

• We are not asking for a logo image like we use in the existing showcase banners. These are typically between 340x140 and 125x75 in size. Also at times i've added a drop shadow under them to make them visible over the screenshot.

• We need to gather what platforms the game is on that use MonoGame. (Like we shouldn't say Desktop if they use XNA or FNA on PC).

• We are asking people to submit issues... but that still requires someone to commit images and add an entry into the game banner data set. Should we simplify things for ourselves and just ask people to make PRs instead?

[JakeLegendXIII]

I think the first and last item here will require some discussion and people to weigh in.

I do have a PR #22 out there for the two middle items if it's okay to address those first. Any suggestions or tweaks are welcome!

For the last one I think that is an option we just have to clearly document the scope of work for someone to add and test the site.