[w3c/manifest] chore: Added PULL_REQUEST_TEMPLATE. (#697)

This change (choose one):

* [ ] Breaks existing normative behavior (please add label "breaking")
* [ ] Adds new normative behavior
* [ ] Makes only editorial changes (only changes informative sections, or
  changes normative sections without changing behavior)
* [X] Is a "chore" (metadata, formatting, fixing warnings, etc).

The following tasks have been completed:

* [X] Confirmed there are no new ReSpec errors/warnings.

Commit message:

This template asks authors to specify whether the change is normative or
editorial, and gather commitment from implementors.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/697

-- Commit Summary --

  * chore: Added PULL_REQUEST_TEMPLATE.

-- File Changes --

    A PULL_REQUEST_TEMPLATE.md (25)

-- Patch Links --

https://github.com/w3c/manifest/pull/697.patch
https://github.com/w3c/manifest/pull/697.diff

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