add template readme, update script

shaders/template/.gitignore

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+potater.exe
+potater

shaders/template/readme.md

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+## Editing Property Templates
+The block, item, and entity property files are generated with a template builder. To modify these templates, you will need [PoTater](https://github.com/Null-MC/PoTater) to regenerate the final property files. Download the latest [release](https://github.com/Null-MC/PoTater/releases) binary for your system, and place it in this `~/template` folder. Then run `~/update.sh` as needed to generate the new property files.

shaders/template/update.sh

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+#!/bin/bash
+
+./potater item   '../item.properties'   '../lib/items.glsl'    -t './item.properties'
+./potater block  '../block.properties'  '../lib/blocks.glsl'   -t './block.properties'
+./potater entity '../entity.properties' '../lib/entities.glsl' -t './entity.properties'