Add documentation on itch.io export issues

[pushfoo]

tl;dr

Exporting for itch.io has some annoying quirks which need documentation.

What needs to be covered

Viewport sizing for desktop

When the desktop view is below a certain width, it itch snaps one of the inner viewport elements down to 600 px in width. If you use the default viewport width of 640 px, the game's viewport does not recenter. It will either be off-center or cut off, depending on how small the browser width is.

The easy fix is setting the viewport width to 600 px in itch's game page options.

The harder fix is to implement #161.

Other items?

Please comment if there's anything else which needs to be explained.

Where to add the doc

docs/ as a separate markdown file.