Add PullRequest templates, with a list of tasks to complete before merging

[kmanning]

I'd like it to be clear for any contributor, what needs to be done before merging a pull request. Let's add a checklist template to PullRequests, so that the PR author can check each task as they're complete.

Things that should be in the list:

• Have you linked this Pull Request to an Issue?
• Have you updated any relevant README files, to explain how this new feature works (with examples)?
• Have you added relevant test cases for your change?
• Do all tests and code style checks pass with ./gradlew check --info?
• Have you successfully run your change in a pipeline in a Jenkins instance?
• Have you updated the CHANGELOG?