Add pull request template (#435)

.github/PULL_REQUEST_TEMPLATE.md

+## Review the Contributing Guidelines
+
+Before submitting a pull request, verify it meets all requirements in the [Contributing Guidelines](https://github.com/donnemartin/system-design-primer/blob/master/CONTRIBUTING.md).
+
+### Translations
+
+See the [Contributing Guidelines](https://github.com/donnemartin/system-design-primer/blob/master/CONTRIBUTING.md).  Verify you've:
+
+* Tagged the [language maintainer](TRANSLATIONS.md)
+* Prefixed the title with a language code
+    * Example: "ja: Fix ..."