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

Author: manuzhang <owenzhang1990@gmail.com>

Closes #118 from manuzhang/add_gh_template.

manuzhang/incubator-gearpump,manuzhang/incubator-gearpump,manuzhang/incubator-gearpump,manuzhang/incubator-gearpump,manuzhang/incubator-gearpump