Document optional test triggers in PR template. Beware, @test-aci or @test-windows in comments will also trigger tests, comments are not ignored for this

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,12 @@
 **What I did**
 
 **Related issue**
-<-- If this is a bug fix, make sure your description includes "fixes #xxxx", or "closes #xxxx" -->
+<!-- If this is a bug fix, make sure your description includes "fixes #xxxx", or "closes #xxxx" -->
+
+<!-- optional tests
+You can add a @ mention to run tests executed by default only on main branch : 
+* `test-aci` to run ACI E2E tests
+* `test-windows` to run tests & E2E tests on windows
+-->
 
 **(not mandatory) A picture of a cute animal, if possible in relation with what you did**