Update A11y doc with Accessibility heading support (#579)

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I submitted the PR for accessibility headers support : #573 . Adding documentation now for the same.
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Pull Request resolved: #579

Test Plan:
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Reviewed By: colriot

Differential Revision: D18725611

Pulled By: muraziz

fbshipit-source-id: 3373e6b99e20f83cde44852ea6e95e7d65f6a2d2

docs/_docs/accessibility.md

@@ -141,3 +141,14 @@ static Component onCreateLayout(
 }
 ```
 
+## Accessibility Heading
+
+Heading is an accessibility property and will help the users to choose to "Navigate by Headings" and ignore scrolling through each and every subitem under a heading. "Navigate based on Heading" can be selected from "Local Context Menu" when talkback is on. Talkback gesture for "Local Context Menu" is **Swipe up then right**.
+Making any component an Accessibility Header is as simple as:
+
+```java
+Row.create(c)
+    .contentDescription("This is an accessibility heading")
+    .accessibilityHeading(true)
+    .build()
+```