Summary:
Followup to fe80a0c0. Use a Markdown extension to let GitHub know this can be rendered as a Preview, otherwise GitHub will display the plaintext version.

[General] [Fixed] - Use Markdown in SUPPORT file
Pull Request resolved: https://github.com/facebook/react-native/pull/23791

Differential Revision: D14349422

Pulled By: hramos

fbshipit-source-id: 836746f35c6e962f3ca64a138bbf764529a6e043