Add guide for PR title (#3936)

* update PR guide for a better release log

* detail on pr title. and fix typo

Author: defcc

.github/CONTRIBUTING.md

@@ -45,6 +45,7 @@ Hi! I’m really excited that you are interested in contributing to Vue.js. Befo
   - Provide convincing reason to add this feature. Ideally you should open a suggestion issue first and have it greenlighted before working on it.
 
 - If fixing a bug:
+  - If you are resolving a special issue, add `(fix #xxxx[,#xxx])` (#xxxx is the issue id) in your PR title for a better release log, e.g. `update entities encoding/decoding (fix #3899)`.
   - Provide detailed description of the bug in the PR. Live demo preferred.
   - Add appropriate test coverage if applicable.