add pull request template
add pull request template

Summary:
Template for pull requests. Derived from Kubernetes'

https://github.com/kubernetes/kubernetes/blob/master/.github/PULL_REQUEST_TEMPLATE.md

Reviewed By: steelcowboy

Differential Revision: D19975620

fbshipit-source-id: 601c78d59f4b8067de96b84a124c731099b2d962

facebook/IT-CPE,facebook/IT-CPE,facebook/IT-CPE