Be sure to do all of the following to help us incorporate your contribution
quickly and easily:

 - [ ] Make sure the commit message is formatted like:
   `[GEARPUMP-<Jira issue #>] Meaningful description of pull request` 
 - [ ] Make sure tests pass via `sbt clean test`.
 - [ ] Make sure old documentation affected by the pull request has been updated and new documentation added for new functionality. 


Add GitHub pull request template
[GEARPUMP-229] Add GitHub pull request template

Author: manuzhang <owenzhang1990@gmail.com>

Closes #118 from manuzhang/add_gh_template.
