Make the pull request template reflect the format being used currently.

Author: fantasai

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,29 +1,3 @@
-^ Please ensure the PR has a brief, yet descriptive name. [Delete this line]
+[css-spec-shortname-1] Brief description which should also include the #issuenum-or-URL and/or link to relevant CSSWG minutes.
 
-Changes related to issue #XXXX
-
-Ready for review by @username
-
-Replace this paragraph with a longer description of the changes made, such that a 
-reviewer can grasp what they will see before they open the diffs.
-
---- Attribution --- [Delete this line and below if inapplicable] ---
-
-If you are not the sole contributor to a contribution (pull request), please identify all
-contributors in the pull request comment.
-
-To add a contributor (other than yourself, that's automatic), mark them one per 
-line as follows:
-
-```
-+@github_username
-```
-
-If you added a contributor by mistake, you can remove them in a comment with:
-
-```
--@github_username
-```
-
-Note: If you are making a pull request on behalf of someone else but you had no 
-part in designing the feature, you can remove yourself with the above syntax.
+Summary should look like the above^. Fill in any additional details here.