github: add template for pull-request description

Commit for #153 introduces changelog and to remind contributors about
changelog entries this commit adds a template for pull-request
description. Learn more about templates on Github in [1].

1. https://docs.github.com/en/github-ae@latest/communities/using-templates-to-encourage-useful-issues-and-pull-requests/creating-a-pull-request-template-for-your-repository

Follows up #121

.github/pull_request_template.md

@@ -0,0 +1,9 @@
+What has been done? Why? What problem is being solved?
+
+I didn't forget about (remove if it is not applicable):
+
+- [ ] Tests (see [documentation](https://pkg.go.dev/testing) for a testing package)
+- [ ] Changelog (see [documentation](https://keepachangelog.com/en/1.0.0/) for changelog format)
+- [ ] Documentation (see [documentation](https://go.dev/blog/godoc) for documentation style guide)
+
+Related issues: