Add pull request template
Add pull request template

This includes links to example search pages, which should reduce the time
to review things and give extra context to new reviewers.  It will hopefully
increase our chance of finding bugs that may not be otherwise caught.

It includes:
- the main search page
- the new statistics finder
- an advanced search topic finder
- a specialist finder
- the Brexit business finder

alphagov/finder-frontend,alphagov/finder-frontend,alphagov/finder-frontend,alphagov/finder-frontend