Update PR template to include checklist (#798)

## Type of change

<!--Delete points that do not apply-->

- Improvement (refactoring, restructuring repository, cleaning tech
debt, ...)

## Changes

The following changes have been made:

- Adds checklist to PR template

Co-authored-by: K1-R1 <77465250+K1-R1@users.noreply.github.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -24,3 +24,14 @@ The following changes have been made:
 <!--Delete everything after the "#" symbol and replace it with a number. No spaces between hash and number-->
 
 Closes #\<issue number\>
+
+## Checklist
+
+- [ ] I have linked to any relevant issues.
+- [ ] I have commented my code, particularly in hard-to-understand areas.
+- [ ] I have updated the documentation where relevant (API docs, the reference, and the Sway book).
+   - [ ] If my change requires substantial documentation changes, I have [requested support from the DevRel team](https://github.com/FuelLabs/devrel-requests/issues/new/choose)
+- [ ] I have added tests that prove my fix is effective or that my feature works.
+- [ ] I have added (or requested a maintainer to add) the necessary `Breaking*` or `New Feature` labels where relevant.
+- [ ] I have done my best to ensure that my PR adheres to [the Fuel Labs Code Review Standards](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md).
+- [ ] I have requested a review from the relevant team or maintainers.