Add the pull request template CONTRIBUTING.md already refers to

Step 7 of "Submitting Pull Requests" tells contributors to open the PR
"filling out the PR template", and there has never been one. The four
things it names -- summary, related issues, screenshots for UI changes,
manual testing -- are the four sections here, plus translations, which
step 8 asks for separately and which is the easiest of the five to
forget: a missing catalog key renders its English source rather than
failing, so nothing in CI or on screen says it was skipped.

Also documents the CI approval gate on fork PRs, now that every outside
contributor's run waits to be started by hand rather than only a
first-time contributor's. Without a note, a contributor whose build
check never appears reads it as an orphaned run and pushes again to
shake it loose, which does nothing.

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,43 @@
+<!--
+  Thanks for contributing to ihasmail. CONTRIBUTING.md has the full guide;
+  this is the short version. Delete any section that does not apply.
+-->
+
+## Summary
+
+<!-- What changes, and why. -->
+
+## Related issues
+
+<!-- e.g. Closes #123. Leave blank if there are none. -->
+
+## Translations
+
+<!--
+  Nine languages ship alongside English, and a missing key silently renders
+  its English source -- so an untranslated string is invisible until somebody
+  reading that language finds it. Say which this PR is, explicitly:
+
+  - Adds or alters user-visible strings: how many keys, and the fallback
+    count before and after.
+  - Adds none.
+
+  "Adds none" is an answer. Saying nothing is not -- it leaves it to be
+  inferred. See CONTRIBUTING.md -> Translations.
+-->
+
+## Testing
+
+<!--
+  What you ran, and what you saw. `npm run typecheck`, `npm test` and
+  `npm run build` all run in CI, so the useful thing here is what CI cannot
+  do: which flows you exercised by hand, and against what -- a real Stalwart
+  instance, or `npm run dev:mock`.
+
+  If the change is visible on screen, drive the built app, not just the
+  store. See CONTRIBUTING.md -> Verifying UI work.
+-->
+
+## Screenshots
+
+<!-- For UI changes. Before/after, or a GIF for anything with motion. -->