Doc: updated pr template to check backward compatibility (#2655)

Co-authored-by: Navin Soni <[email protected]>

Author: shreyapandit

.github/PULL_REQUEST_TEMPLATE.md

@@ -11,13 +11,15 @@ _Put an `x` in the boxes that apply. You can also fill these out after creating
 #### General
 
 - [ ] I have read the [CONTRIBUTING](https://github.com/aws/sagemaker-python-sdk/blob/master/CONTRIBUTING.md) doc
+- [ ] I certify that the changes I am introducing will be backword compatible, and I have discussed concerns about this, if any, with the Python SDK team
 - [ ] I used the commit message format described in [CONTRIBUTING](https://github.com/aws/sagemaker-python-sdk/blob/master/CONTRIBUTING.md#committing-your-change)
 - [ ] I have passed the region in to all S3 and STS clients that I've initialized as part of this change.
 - [ ] I have updated any necessary documentation, including [READMEs](https://github.com/aws/sagemaker-python-sdk/blob/master/README.rst) and [API docs](https://github.com/aws/sagemaker-python-sdk/tree/master/doc) (if appropriate)
 
 #### Tests
 
 - [ ] I have added tests that prove my fix is effective or that my feature works (if appropriate)
+- [ ] I have added unit and/or integration tests as appropriate to ensure backward compatibility of the changes
 - [ ] I have checked that my tests are not configured for a specific region or account (if appropriate)
 - [ ] I have used [`unique_name_from_base`](https://github.com/aws/sagemaker-python-sdk/blob/master/src/sagemaker/utils.py#L77) to create resource names in integ tests (if appropriate)