JSON & TMX Format Documentation is Missing the Maths

Nobody has claimed this yet.

The math behind how to derive the X and Y offset for a tile is missing from the documentation. While anyone with a computer science degree or anyone that has been doing game programming for a while should be able to just figure this out, this does leave a gap of information for people that haven't been doing this.

I propose adding a math section the breaks down the process maybe. Something in the following format.

## Normalizing the (tile id) GID
`normalized_tile_id = tile_id - firstgid`
## Solving for the X Offset
`normalized_tile_id * tilewidth % tileset_width`
## Solving for the Y Offset
`Math.floor( ( normalized_tile_id * tilewidth ) / tileset_width) * tileheight`
## Considerations for Margins
_not 100% sure on the math for this_

Something like this would be helpful to anyone that just wants to be able to parse in maps made with Tiled because it lowers the threshold to entry, which in turn makes it more likely for support to be more wide-spread.

Locate the JSON and TMX format documentation and identify where tile GID normalization and tile offsets are explained. Add a math section covering the proposed X and Y calculations, then verify the formulas and document how margins affect them; the documentation should let readers derive offsets without prior game-programming experience.

Written by the indexing model from the issue text.