making_a_pull_request: describe how to use the pull request template

Fixes issue #36.

- Use slightly deceptive quoting to imply the file name for which they
  are looking.
- Describe why they will want to use it.
- Tell them how to use it.
Merge pull request #37 from mwmiller/pull_request_template

making_a_pull_request: describe how to use the pull request template