Add item about referencing PRs in PRs to master

[evansiroky]

Checklist

• Appropriate branch selected (all PRs must first be merged to dev before they can be merged to master)
• Any modified or new methods or classes have helpful JavaDoc and code is thoroughly commented

N/A - just changes the PR template

• The description lists all applicable issues this PR seeks to resolve
• The description lists any configuration setting(s) that differ from the default settings

N/A

• All tests and CI builds passing

Description

This PR seeks to improve the PR template by asking for relevant PRs to be included in descriptions when merging to master.

[codecov-io]

Codecov Report

Merging #175 into dev will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##               dev    #175   +/-   ##
=======================================
  Coverage     5.14%   5.14%           
  Complexity      94      94           
=======================================
  Files          133     133           
  Lines         6926    6926           
  Branches       908     908           
=======================================
  Hits           356     356           
  Misses        6542    6542           
  Partials        28      28

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[landonreed]

🎉 This PR is included in version 3.4.0 🎉

The release is available on GitHub release

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