Update PR template

Update pull request template for more details.

Signed-off-by: Nguyen Quang Huy <huynq0911@gmail.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -2,10 +2,14 @@
 1. Make sure to read the Contributing Guide before submitting your PR: https://github.com/helm/helm/blob/master/CONTRIBUTING.md
 2. If this PR closes another issue, add 'closes #<issue number>' somewhere in the PR summary. GitHub will automatically close that issue when this PR gets merged. Alternatively, adding 'refs #<issue number>' will not close the issue, but help provide the reviewer more context.-->
 
+**Which issue(s) this PR fixes**:
+
 **What this PR does / why we need it**:
 
 **Special notes for your reviewer**:
 
+**Docs Changes**:
+
 **If applicable**:
 - [ ] this PR contains documentation
 - [ ] this PR contains unit tests