Create pull_request_template.md

Closes #78 

Signed-off-by: Lukáš Vlček <lukas.vlcek@aiven.io>

.github/pull_request_template.md

@@ -0,0 +1,7 @@
+## Description
+
+Describe your PR and add links to relevant issues.
+
+---
+
+- [ ] All my commits include DCO.<br>_DCO stands for **Developer Certificate of Origin** and it is your declaration that your contribution is correctly attributed and licensed. Please read more about how to attach DCO to your commits [here](https://github.com/aiven/prometheus-exporter-plugin-for-opensearch/blob/main/CONTRIBUTING.md#developer-certificate-of-origin) (spoiler alert: in most cases it is as simple as using `-s` option when doing `git commit`).<br>Please be aware that commits without DCO will cause failure of PR CI workflow and can not be merged._