External icon library

.gitattributes

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+*.svg linguist-generated
+index.json linguist-generated

README.md

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 - `state` generally refers to the state pattern.
 - All state transition and handling should be shynchronous.
     - TODO: To handle something asynchronously, create dedicated state blocking canvas events.
+
+## External icons
+- Put icon files at `public/shapes` directory.
+- Run `scripts/shape_icons.js` to generate `index.json` that contains index information of icons.
+
+```
+$ node ./scripts/shape_icons.js ./public/shapes/TARGET_DIRECTORY
+```
+
+- Directory structure is reflected to UI as it is.
+- The script generates unique id for each icon. Those ids change every time it runs.