chore: add PULL_REQUEST_TEMPLATE.md (#121)

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,23 @@
+Closes #???
+
+Person merging, please make sure that commits are squashed with one of the following as a commit message prefix:
+
+- chore: fixes unrelated to the spec itself (e.g., fix to Github action, html tidy, spec config, etc.). 
+- editorial: a non-normative change to the spec (e.g., fixing an example or typo, adding a note).
+- change: a normative change to existing engine behavior.
+- And use none if it's a new normative requirement. 
+
+If this is a "change", please explain what's significantly changing. 
+In particular, if the change results in potential backwards compatibility issues and content breakage, it needs to be justified.
+
+For normative spec changes, please get implementation commitments anf file issues on the various engines during the review process. All tasks should be complete before merging. 
+
+Implementation commitment - primarily for "change" and other normative changes:
+
+- [ ] [WebKit](https://bugs.webkit.org/???)
+- [ ] [Chromium](https://bugs.chromium.org/???)
+- [ ] [Gecko](http://bugzilla.mozilla.org/???)
+
+Tasks:
+
+- [ ] [Added tests](https://github.com/web-platform-tests/wpt/pulls/???)