Update PULL_REQUEST_TEMPLATE to request detailed information

[apcraig]

PR checklist

• [X] Short (1 sentence) summary of your PR: Update PULL_REQUEST_TEMPLATE to request detailed information
• [X] Developer(s): apcraig
• [X] Suggest PR reviewers from list in the column to the right.
• [X] Please copy the PR test results link or provide a summary of testing completed below. No testing done
• How much do the PR code changes differ from the unmodified code?
  • [X] bit for bit
  • [ ] different at roundoff level
  • [ ] more substantial
• Does this PR create or have dependencies on Icepack or any other models?
  • [ ] Yes
  • [X] No
• Does this PR update the Icepack submodule? If so, the Icepack submodule must point to a hash on Icepack's main branch.
  • [ ] Yes
  • [X] No
• Does this PR add any new test cases?
  • [ ] Yes
  • [X] No
• Is the documentation being updated? ("Documentation" includes information on the wiki or in the .rst files from doc/source/, which are used to create the online technical docs at https://readthedocs.org/projects/cice-consortium-cice/. A test build of the technical docs will be performed as part of the PR testing.)
  • [ ] Yes
  • [X] No, does the documentation need to be updated at a later time?
    • [ ] Yes
    • [X] No
• [X] Please provide any additional information or relevant details below:

Update PULL_REQUEST_TEMPLATE to request detailed information about changes associated with the PR. This will be useful for the commit log when squash merging the PR.

Closes #930

[apcraig]

Please let me know if you prefer different wording. Once we converge on changes to the template, I'll do the same for Icepack.