docs: Update PR template

[JST5000]

High Level Overview of Change

Updating the CHANGELOG is often forgotten by maintainers and external contributors, so we should add it to our PR template.

Context of Change

This should make it easier to contribute to the repo since it's one less back and forth we'll need to message folks about, and make it easier to guarantee our changes are tracked for releases.

Type of Change

• [X] Documentation Updates

Did you update CHANGELOG.md?

• [X] No, this change does not impact library users

[ckniffen]

This has already helped me out with my latest PR to xrpl.js