Add environment variable reminder

Author: micahalcorn

.github/PULL_REQUEST_TEMPLATE.md

@@ -2,9 +2,10 @@ First pull request? Read our [guide to contributing](http://docs.originprotocol.
 
 ### Checklist:
 
-- [ ] Test your work and double check you didn't break anything
-- [ ] Update any relevant READMEs and [docs](https://github.com/OriginProtocol/docs)
+- [ ] Test your work and double-check to confirm that you didn't break anything
 - [ ] Wrap any new text/strings for translation
+- [ ] Map any new environment variables with a default value in the Webpack config
+- [ ] Update any relevant READMEs and [docs](https://github.com/OriginProtocol/docs)
 
 ### Description: