add readme for u0456-slab

Author: chrissimpkins

glyphs/u0456-slab/README.md

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+<img src="https://user-images.githubusercontent.com/4249591/32996082-154511dc-cd4c-11e7-9a21-62d207378107.png" height="60">
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+<img src="https://user-images.githubusercontent.com/4249591/32996084-16fe9e12-cd4c-11e7-8f47-aa0ab9f59160.png" height="60">
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+<img src="https://user-images.githubusercontent.com/4249591/32996085-18e0e794-cd4c-11e7-892a-35bad33004c2.png" height="60">
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+<img src="https://user-images.githubusercontent.com/4249591/32996086-1af0eb92-cd4c-11e7-85a0-8648ee023570.png" height="60">