Update pull request template (#2402)

## Description:

Adds a section to the pull request template about linking issues, as
suggested in #2401

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Please put your Discord username so you can be contacted if a bug or
regression is found:

loymdayddaud

Author: TheGiraffe3

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,3 +1,6 @@
+If this PR fixes an issue, link it below. If not, delete these two lines.
+Resolves #(issue number)
+
 ## Description:
 
 Describe the PR.