Add spritezero icons and instructions (#832)

basemap/assets/README.md

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+# OpenStreetMap-Carto Icons
+
+The icons stored in this directory come from the [OpenStreetMap-Carto](https://github.com/gravitystorm/openstreetmap-carto/)
+project released under the CC0 Public Domain Dedication, version 1.0, as published by Creative Commons.
+
+Created by Andy Allan <andy@gravitystorm.co.uk> and contributors.
+Based on original cartography by Steve Chilton and others.
+
+Both spritezero and spreet can be used to generate the png and json files from the SVG files.
+
+## Spritezero
+
+[Spritezero](https://github.com/elastic/spritezero#readme), while unmaintained, is currently the preferred tool to generate the png and json files from the SVG files.
+
+```
+volta install node@10
+npm install -g @elastic/spritezero-cli
+spritezero --ratio=1 --sdf icons@1x icons
+spritezero --ratio=1 --sdf icons@2x icons
+spritezero --ratio=1 --sdf icons@4x icons
+```
+
+We fake ratio 2 and 4 as spritezero's generated files are broken.
+
+## Spreet
+
+[Spreet](https://github.com/flother/spreet) is a promising alternative to spritezero, and we hope to use it in the future.
+However, the generated files are not yet as good as the ones generated by spritezero.
+
+```
+cargo install spreet
+spreet --sdf --ratio 1 icons icons@1x
+spreet --sdf --ratio 2 icons icons@2x
+spreet --sdf --ratio 4 icons icons@4x
+```