core: update github PR template (#2377)

Co-authored-by: Wesley Rosenblum <55108558+WesleyRosenblum@users.noreply.github.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,10 +1,13 @@
+### Release Summary:
+<!-- If this is a feature or bug that impacts customers and is significant enough to include in the "Summary" section of the next version release, please include a brief (1-2 sentences) description of the change. The audience of this summary is customers, not maintainers or reviewers. See https://github.com/aws/s2n-tls/releases/tag/v1.5.7 for an example. Otherwise, leave this section blank -->
+
 ### Resolved issues:
 
 resolves #ISSUE-NUMBER1, resolves #ISSUE-NUMBER2, etc.
 
 ### Description of changes: 
 
-<!-- Describe s2n-quic’s current behavior and how your code changes that behavior. If there are no issues this pr is resolving, explain why this change is necessary.-->
+<!-- Describe s2n-quic’s current behavior and how your code changes that behavior. If there are no issues this pr is resolving, explain why this change is necessary. If a callout is specific to a section of code, it might make more sense to leave a comment on your own PR file diff. -->
 
 ### Call-outs:
 
@@ -13,8 +16,9 @@ resolves #ISSUE-NUMBER1, resolves #ISSUE-NUMBER2, etc.
 ### Testing:
 
 <!--How is this change tested (unit tests, fuzz tests, etc.)? Are there any testing steps to be verified by the reviewer?
-
+How can you convince your reviewers that this PR is safe and effective?
 Is this a refactor change? If so, how have you proved that the intended behavior hasn't changed? -->
 
+
 By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.