Add Yannick to contributors

[mcgibbon]

Purpose

When merging Yannick's changes for enabling different number of ranks on tile edges/corners, we didn't add him to the CONTRIBUTORS.md. This PR does so.

Checklist

Before submitting this PR, please make sure:

• You have followed the coding standards guidelines established at Code Review Checklist.
• Docstrings and type hints are added to new and updated routines, as appropriate
• All relevant documentation has been updated or added (e.g. README, CONTRIBUTING docs)
• For each public change and fix in pace-util, HISTORY has been updated
• Unit tests are added or updated for non-stencil code changes

Additionally, if this PR contains code authored by new contributors:

• The names of all the new contributors have been added to CONTRIBUTORS.md

[elynnwu]

It would be nice if we don't have to run all tests for a change like this...

[mcgibbon]

launch jenkins