Added pull request template (#723)

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,11 @@
+## Description
+<!-- Goal of the pull request -->
+
+
+## Related issues / Pull Requests
+<!-- Fixes #xyz, Contributes to #xyz, Related to #xyz-->
+
+
+## Contributor Agreement
+
+- [ ] I confirm I have signed the [Contributor License Agreement](https://github.com/overleaf/overleaf/blob/master/CONTRIBUTING.md#contributor-license-agreement)