Update A11y doc with Accessibility heading support (#579)
Update A11y doc with Accessibility heading support (#579)

Summary:
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I submitted the PR for accessibility headers support : https://github.com/facebook/litho/pull/573 . Adding documentation now for the same.
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## Changelog

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Pull Request resolved: https://github.com/facebook/litho/pull/579

Test Plan:
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Reviewed By: colriot

Differential Revision: D18725611

Pulled By: muraziz

fbshipit-source-id: 3373e6b99e20f83cde44852ea6e95e7d65f6a2d2

facebook/litho,facebook/litho,facebook/litho,facebook/litho,facebook/litho,facebook/litho