Add pull request template 

Add pull request template on GitHub. Refer to WRF-Chem guidance for this.
https://github.com/wrf-model/WRF/wiki/Making-a-good-pull-request-message

Title
Type (e.g., bug fix, enhancement)
Keywords
Source/Contributor
Description of changes
      Problem:
      Solution:
Issue (does it resolve specific github issue)
List of modified files
Test conducted
Release note
