Pull Request Template: rename and clarify how to use.
Pull Request Template: update internal link, minor text changes

- Update internal link in CONTRIBUTING.md for new file name
- Update text slightly:
  - Replace repeated 'help' used in two ways in opening sentence
  - Add a descriptor for what they are copying.

Did a recursive search over the repo to make sure there were't any
other references to the old file name. Thanks to @moollaza for spotting
this one!
Merge pull request #341 from mwmiller/pull_request_template

Pull Request Template: rename and clarify how to use.