[ftr] add pull_request_template.md

[krHERO]

Is your feature request related to a problem? Please describe.
Despite github recommendations for community standards the edirom-online repo does not have a pull_request_template.md yet.
It seems useful to have a standard process for every PR, to have an overview/list for the author to check before requesting a review and to capture some standard information to make the review-process easier.

Describe the solution you'd like
Examine several examples of pull requests and choose one to adapt (with some adjustments).

[krHERO]

this issue is closed with PR #407