github: add pull request checklist

Author: DifferentialOrange

.github/pull_request_template.md

@@ -0,0 +1,16 @@
+If this pull request introduces support for new metrics, do not forget to
+- [ ] add Grafana panel;
+- [ ] run `make update-tests` to build new dashboard artifacts;
+- [ ] add example application code to generate non-null metrics;
+- [ ] attach a screenshot of a new panel to the PR description;
+- [ ] add new Telegraf tag key, if relevant,
+  - [ ] to all `telegraf.conf` files,
+  - [ ] to "Grafana dashboard" documentation page;
+- [ ] add alert example, if relevant,
+  - [ ] to `alerts.yml`,
+  - [ ] to "Alerting" documentation page;
+- [ ] update [Metrics support wiki page](https://github.com/tarantool/grafana-dashboard/wiki/Metrics-support) after merging.
+
+In any case, do not forget to
+- [ ] link an issue,
+- [ ] add a CHANGELOG entry.