Add RO-Crate sticker templates

docs/assets/img/sticker-templates/README.md

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+# RO-Crate sticker templates
+
+If you do a lot of in-person RO-Crate outreach, you may want to get some stickers printed to distribute!
+
+The templates in this folder conform to the widely-adopted [Stickers Standard](https://sticker.how) for hexagonal stickers:
+
+> A default hexagon sticker must be represented by a regular hexagon where the largest diagonals should measure 5.08 centimeters, and which must remain within 1 millimeter tolerance. The sticker must be oriented with a vertex positioned at the top.
+
+When printing stickers, ensure you follow the size guideline above to enable maximum interoperability with other hexagonal stickers.
+
+There are two versions of the sticker:
+* with bleed - the border is thicker, giving an extra margin to allow for slightly inaccurate cutting during printing. In this case the bleed is about 2mm. File: [ro-crate-hex-with-bleed.svg](ro-crate-hex-with-bleed.svg)
+* without bleed - how the sticker should appear once printed. File: [ro-crate-hex.svg](ro-crate-hex.svg)
+
+In both cases, the SVG contains a "Preview without bleed" layer which can be toggled to view the other version.
+
+When communicating with your printer, make clear if you are sending the version with or without bleed.
+
+This folder also contains a PNG rendering of the sticker for ease of online viewing: [ro-crate-hex.png](ro-crate-hex.png)
+
+![RO-Crate logo inside a teal hexagonal border](ro-crate-hex.png)