Contribution: Pull Request Descriptions

This pull request clarifies the pull request part of the description of our contribution guide lines.

@ComputationalRadiationPhysics/picongpu-developers 
@ComputationalRadiationPhysics/picongpu-maintainers 

Please provide an **expressive** descriptions in your pull requests.
Together with the labels and linked (!) issues (if one was there) they build the backbone for writing release notes and to backport changes to older versions.

### Additions

- pull request descriptions are, besides issues, an extremely
  important reference for the change log
- one maintainer for coordination usually works out for small
  changes, one should try to let other developers have a second
  look, too
- bigger changes should at least try to get two persons in the review
What did you implement and why?