add PULL_REQUEST_TEMPLATE.md file

[wholmgren]

I'd like to make a .github directory with a PULL_REQUEST_TEMPLATE.md file. The XArray/Pandas template would work for me:

 - [ ] Closes #xxxx
 - [ ] Tests added / passed
 - [ ] Passes ``git diff upstream/master | flake8 --diff``
 - [ ] Fully documented, including `whats-new.rst` for all changes and `api.rst` for new API

https://help.github.com/articles/creating-a-pull-request-template-for-your-repository/

Later, we might consider adding a CONTRIBUTING.md, ISSUE_TEMPLATE.md, or CODE_OF_CONDUCT.md files to the .github directory. I'll leave those to separate issues.

[markcampanelli]

If pvlib were >=1.0.0, then I would suggest a SemVer-related checklist. I'm not sure if we can capture the minor vs. patch guidelines at this point in pre-1.0.0 releases.

[wholmgren]

closed by #355