Add GitHub Pull Request Template (#328)

Co-authored-by: Marcos Cáceres <marcos@marcosc.com>
Co-authored-by: Filip Skokan <panva.ip@gmail.com>

Author: 3 people

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,20 @@
+Closes #???
+
+The following implementers have shown interest:
+
+ * …
+ * …
+
+The following tasks have been completed:
+
+ * [ ] Modified Web platform tests (link to pull request)
+
+Implementation issues:
+
+ * [ ] Chromium (https://bugs.chromium.org/p/chromium/issues/detail?id=)
+ * [ ] Gecko (https://bugzilla.mozilla.org/show_bug.cgi?id=)
+ * [ ] WebKit (https://bugs.webkit.org/show_bug.cgi?id=)
+ * [ ] Deno (https://github.com/denoland/deno/issues/)
+ * [ ] Node.js (https://github.com/nodejs/node/issues/)
+ * [ ] workerd (https://github.com/cloudflare/workerd/issues/)
+ * [ ] Vercel Edge Runtime (https://github.com/vercel/edge-runtime/issues/)