add translation info to PR template

Author: alexcjohnson

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,5 +1,15 @@
 Thanks for your interest in plotly.js!
 
+### Translations:
+
+- Make a PR directly to the main repository
+- Label it `type: translation`
+- Please @ mention a few other speakers of this language who can help review your translations.
+- If you've omitted any keys from [dist/translation_keys.txt](https://github.com/plotly/plotly.js/blob/master/dist/translation-keys.txt) - which means they will fall back on the US English text - just make a short comment about why in the PR description: the English text works fine in your language, or you would like someone else to help translating those, or whatever the reason.
+- You should only update files in `lib/locales/`, not those in `dist/`
+
+### Features, Bug fixes, and others:
+
 Developers are strongly encouraged to first make a PR to their own plotly.js fork and ask one of the maintainers to review the modifications there. Once the pull request is deemed satisfactory, the developer will be asked to make a pull request to the main plotly.js repo and may be asked to squash some commits before doing so.
 
 Before opening a pull request, developer should: