Add pull request templates

[chippmann]

Describe the problem or limitation you are having in your project:
To make pull requests more consistent, we should provide a pull request template.

Describe how this feature / enhancement will help you overcome this problem or limitation:
It directly related to #11 but can be done separately.

Show a mock up screenshots/video or a flow diagram explaining how your proposal will work:
None to provide

Describe implementation detail for your proposal (in code), if possible:
The content of this template needs to be discussed but should definitely contain a brief overview over the changes and why they are done.

If this enhancement will not be used often, can it be worked around with a few lines of code?:
It's not completely necessary but definitely helpful.

Is there a reason why this should be in this project and not individually solved?:
As pull requests are made in this project.