* Create pull_request_template.md

Pull Request Template containing the main points that should be taken into account when submitting a new request.

* Update pull_request_template.md

* Moved meta points to comment section
* Removed pointer to Contribution Guidelines (shown by Github, anyways)
* Added mvn:install hint to checklist

Co-authored-by: Johannes Eschrig <johannes.eschrig@sap.com>