pull request: add required info

Please use these 2 sections for describing a pull request. They should be part
of every pull request.

The type specifies what is expected from the pull request and when it can be
released. For instance a feature pull request can't be expected to go to the
patch release.

Important: do not mix pull request types !

Changed also the heading type, to make it smaller.

.github/pull_request_template.md

@@ -1,11 +1,21 @@
-# Description
+### Description
 
-> Detailed changes summary | testing | dependencies
-> Good example: https://os.mbed.com/docs/latest/reference/guidelines.html#workflow (Pull request template)
+<!-- 
+    Required
+    Add here detailed changes summary, testing results, dependencies 
+    Good example: https://os.mbed.com/docs/latest/reference/guidelines.html#workflow (Pull request template)
+-->
 
-# Pull request type
+
+### Pull request type
+
+<!-- 
+    Required
+    Please tick one of the following types 
+-->
 
 - [ ] Fix
 - [ ] Refactor
-- [ ] New Target
+- [ ] New target
 - [ ] Feature
+- [ ] Breaking change