Use long sentences for pull request template
Use long sentences for pull request template

Address #568

Trevor Keller thinks that long sentences read better so changed to
oblige.

usnistgov/chimad-phase-field,wd15/chimad-phase-field,wd15/chimad-phase-field,usnistgov/chimad-phase-field,wd15/chimad-phase-field,usnistgov/chimad-phase-field