Update ISSUE_TEMPLATE.md

Summary:
Provide a sample new issue template to encourage people to provide additional detail as needed. This should help us reduce the number of new issues that are opened without sufficient information or actionable requests.

The minimal use of markdown is intentional, as these guidelines are more likely to be found in raw text inside the [new issue composer](https://github.com/facebook/react-native/issues/new) rather than rendered to HTML.
Closes https://github.com/facebook/react-native/pull/9074

Differential Revision: D3785606

Pulled By: hramos

fbshipit-source-id: 9982566a0f0d86c71eeface33bd0624a4a9935b2