Create a PULL_REQUEST_TEMPLATE

#### Issue Summary

We would like to have a template for describing what has changed in the pull request being submitted.

[Github docs on this](https://help.github.com/articles/creating-a-pull-request-template-for-your-repository/) 

You can use [this file](https://github.com/sendgrid/open-source-library-data-collector/blob/master/.github/PULL_REQUEST_TEMPLATE.md) as an example