add readme for u0457-slab

Author: chrissimpkins

glyphs/u0457-slab/README.md

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+<img src="https://user-images.githubusercontent.com/4249591/32996150-0422b778-cd4d-11e7-8f84-d7a5a7c2adf7.png" height="60">
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+<img src="https://user-images.githubusercontent.com/4249591/32996151-06122ed8-cd4d-11e7-8af9-4ce0f2136d59.png" height="60">
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+<img src="https://user-images.githubusercontent.com/4249591/32996153-081815ee-cd4d-11e7-9b2e-8a091fba7263.png" height="60">
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+<img src="https://user-images.githubusercontent.com/4249591/32996155-09ffacaa-cd4d-11e7-9a1b-592fcc2ffc1e.png" height="60">