Add a PR Template

[kevinfodness]

Description

Add a GitHub template for pull requests that prompts for relevant information but doesn't require too much of the author. Should be helpful and not just red tape. Intended audience is the user of a repo created from this one to manage a project, not for contributions to this repo itself (so it shouldn't be aimed at open source contributors to create-wordpress-project but rather people building a site in a repo created from this template). We may want to name the file differently so it doesn't come up when someone creates a PR in create-wordpress-project, but is renamed as part of the installer script.

Use Case

When a user uses this repo to create a new repo for their project, they should be able to get some helpful prompts when creating new pull requests.