Request translation changes via Transifex (#2157)

Some of our users submit direct PRs against our translations. This update to our PR template is meant to encourage them to contribute translations via Transifex.

Prompted by this user PR: https://github.com/signalapp/Signal-Desktop/pull/2156

.github/PULL_REQUEST_TEMPLATE.md

@@ -13,6 +13,7 @@ Remember, you can preview this before saving it.
 
 
 ### Contributor checklist:
+- [ ] My contribution is **not** related to translations. _Please submit translation changes via our [Signal Desktop Transifex project](https://www.transifex.com/liliakai/signal-desktop/)._
 - [ ] My commits are in nice logical chunks with [good commit messages](http://chris.beams.io/posts/git-commit/)
 - [ ] My changes are [rebased](https://medium.freecodecamp.org/git-rebase-and-the-golden-rule-explained-70715eccc372) on the latest [`development`](https://github.com/WhisperSystems/Signal-Desktop/tree/development) branch
 - [ ] My changes pass 100% of [local tests](https://github.com/WhisperSystems/Signal-Desktop/blob/master/CONTRIBUTING.md#tests)