Create a pull request template

[williamjamir]

This repository needs to have a PR template for whenever someone opens a PR a series of checklists is displayed, with the following items:

• Make sure to update the changelog
• Update the filled_case_description.py file located on tests/common_testing/alfasim_sdk_common_testing/filled_case_descriptions.py
  whenever a change on CaseDescription occurs. This modification is necessary in order to execute a series of roundtrip tests.
• Check if the documentation needs to be updated, or if needs to mention this change.
• If a new class is added or renamed, make sure to update __init__.py from the root directory reflects this change.

Reference:
https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/creating-a-pull-request-template-for-your-repository