Add link to coding style documentation (#4280)

.github/PULL_REQUEST_TEMPLATE/pull_request_template.md

@@ -23,7 +23,7 @@
 ## Checklist:
 <!--- Go over all the following points, and put an `x` in all the boxes that apply. -->
 <!--- If you're unsure about any of these, don't hesitate to ask. We're here to help! -->
-- [ ] My code follows the code style of this project.
+- [ ] My code follows the **[CODING STYLE of this project](https://docs.pjsip.org/en/latest/get-started/coding-style.html)**.
 - [ ] My change requires a change to the documentation.
 - [ ] I have updated the documentation accordingly.
 - [ ] I have read the **CONTRIBUTING** document.