chore: add "Release note" to PR template (#39)

Author: guewa

.github/pull_request_template.md

@@ -4,3 +4,17 @@
 Fixes #
 
 **Special notes for your reviewer**:
+
+**Release note**:
+<!--  Write your release note:
+1. Enter your release note in the below block.
+2. If no release note is required, just write "NONE" within the block.
+
+Format of block header: <category> <target_group>
+Possible values:
+- category:       breaking|feature|bugfix|doc|other
+- target_group:   user|operator|developer|dependency
+-->
+```feature user
+
+```