Add PR template (zalando#1234)

* Add PR template

* move to github folder and rename file

* add headlines

Author: FxKu

.github/PULL_REQUEST_TEMPLATE/postgres-operator-pull-request-template.md

@@ -0,0 +1,18 @@
+## Problem description
+
+
+
+## Linked issues
+
+
+
+## Checklist
+
+Thanks for submitting a pull request to the Postgres Operator project.
+Please, ensure your contribution matches the following items:
+
+- [ ] Your go code is [formatted](https://blog.golang.org/gofmt). Your IDE should do it automatically for you.
+- [ ] You have updated [generated code](https://github.com/zalando/postgres-operator/blob/master/docs/developer.md#code-generation) when introducing new fields to the `acid.zalan.do` api package.
+- [ ] New [configuration options](https://github.com/zalando/postgres-operator/blob/master/docs/developer.md#introduce-additional-configuration-parameters) are reflected in CRD validation, helm charts and sample manifests.
+- [ ] New functionality is covered by [unit](https://github.com/zalando/postgres-operator/blob/master/docs/developer.md#unit-tests) and/or [e2e](https://github.com/zalando/postgres-operator/blob/master/docs/developer.md#end-to-end-tests) tests.
+- [ ] You have checked existing open PRs for possible overlay and referenced them.