Add release notes section to PR template

Author: orenc17

.github/pull_request_template.md

@@ -1,15 +1,15 @@
 ### Description
 
-<!-- 
+<!--
     Required
-    Add here detailed changes summary, testing results, dependencies 
+    Add here detailed changes summary, testing results, dependencies
     Good example: https://os.mbed.com/docs/mbed-os/latest/contributing/workflow.html (Pull request template)
 -->
 
 
 ### Pull request type
 
-<!-- 
+<!--
     Required
     Please add only one X to one of the following types. Do not fill multiple types (split the pull request otherwise).
     Please note this is not a GitHub task list, indenting the boxes or changing the format to add a '.' or '*' in front
@@ -26,8 +26,15 @@
 
 ### Reviewers
 
-<!-- 
+<!--
     Optional
     Request additional reviewers with @username
 -->
 
+### Release Notes
+
+<!--
+    Optional
+    In case of breaking changes, functionality changes or refactors, please add release notes here. 
+    For more information, please see [the contributing guidelines](https://os.mbed.com/docs/mbed-os/latest/contributing   /workflow.html#pull-request-types).
+-->