Update PULL_REQUEST_TEMPLATE.md

[MasterJ93]

Description

Added links to the Swift API Design Guidelines and ATProtoKit API Design Guidelines.

Type of Change

• [ ] Bug Fix
• [ ] New Feature
• [x] Documentation

Checklist:

• [x] My code follows the ATProtoKit API Design Guidelines as well as the Swift API Design Guidelines.
• [x] I have performed a self-review of my own code and commented it, particularly in hard-to-understand areas.
• [x] I have made corresponding changes to the documentation.
• [x] My changes generate no new warnings or errors in the compiler or runtime.
• [x] My code is able to build and run on my machine.