Add a basic PR checklist for all PRs (element-hq#389)

* Add a basic PR checklist for all PRs

It'll be mildly annoying for core developers who have to constantly remove or edit this, but it'll also serve as a good reminder to do these things.

Note that signoff is not required for core developers.

* Update .github/PULL_REQUEST_TEMPLATE.md

Author: turt2live

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,13 @@
+<!-- Thanks for submitting a PR! Please ensure the following requirements are met in order for us to review your PR -->
+
+## Checklist
+
+* [ ] Ensure your code works with manual testing
+* [ ] Linter and other CI checks pass
+* [ ] Sign-off given on the changes (see [CONTRIBUTING.md](https://github.com/vector-im/element-desktop/blob/develop/CONTRIBUTING.md))
+
+<!--
+If you would like to specify text for the changelog entry other than your PR title, add the following:
+
+Notes: Add super cool feature
+-->