chore: update github PR template (#4885)

Author: lrstewart

Diff for: .github/PULL_REQUEST_TEMPLATE.md

@@ -1,19 +1,30 @@
+### 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 future 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.
+resolves #ISSUE-NUMBER1, resolves #ISSUE-NUMBER2, etc.
 
 ### Description of changes: 
 
 Describe s2n’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.
 
 ### Call-outs:
 
-Address any potentially confusing code. Is there code added that needs to be cleaned up later? Is there code that is missing because it’s still in development? 
+Address any potentially confusing code. Is there code added that needs to be cleaned up later? Is there code that is missing because it’s still in development? 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.
 
 ### Testing:
 
-How is this change tested (unit tests, fuzz tests, etc.)? Are there any testing steps to be verified by the reviewer?
-
+How is this change tested (unit tests, fuzz tests, etc.)? What manual testing was performed? 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?
 
+Remember:
+* Any change to the library source code should at least include unit tests.
+* Any change to the core stuffer or blob methods should include [CBMC proofs](https://github.com/aws/s2n-tls/tree/main/tests/cbmc).
+* Any change to the CI or tests should:
+   1. prove that the test succeeds for good input
+   2. prove that the test fails for bad input (eg, a test for memory leaks fails when a memory leak is committed)
+
+
 By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.