Update PR template to add integration tests in checklist (#2306)

Author: hawflau

.github/PULL_REQUEST_TEMPLATE.md

@@ -6,10 +6,11 @@
 
 *Checklist:*
 
-- [ ] Add/update tests using:
+- [ ] Add/update [unit tests](https://github.com/aws/serverless-application-model/blob/develop/DEVELOPMENT_GUIDE.md#unit-testing-with-multiple-python-versions) using:
     - [ ] Correct values
     - [ ] Bad/wrong values (None, empty, wrong type, length, etc.)
     - [ ] [Intrinsic Functions](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/intrinsic-function-reference.html)
+- [ ] Add/update [integration tests](https://github.com/aws/serverless-application-model/blob/develop/INTEGRATION_TESTS.md)
 - [ ] `make pr` passes
 - [ ] Update documentation
 - [ ] Verify transformed template deploys and application functions as expected