Add a basic PR checklist for all PRs (#2511)

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.

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,7 +1,13 @@
-<!-- Please read https://github.com/matrix-org/matrix-js-sdk/blob/develop/CONTRIBUTING.md before submitting your pull request -->
+<!-- Thanks for submitting a PR! Please ensure the following requirements are met in order for us to review your PR -->
 
-<!-- Include a Sign-Off as described in https://github.com/matrix-org/matrix-js-sdk/blob/develop/CONTRIBUTING.md#sign-off -->
+## Checklist
 
-<!-- To specify text for the changelog entry (otherwise the PR title will be used):
-Notes:
+* [ ] Tests written for new code (and old code if feasible)
+* [ ] Linter and other CI checks pass
+* [ ] Sign-off given on the changes (see [CONTRIBUTING.md](https://github.com/matrix-org/matrix-js-sdk/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
 -->