Update CONTRIBUTING.md

[manez]

GitHub Issue: Islandora/documentation#1588

What does this Pull Request do?

Adds mention of the new User Call to CONTRIBUTING.md.

How should this be tested?

Preview & review

Interested parties

@Islandora/8-x-committers

[codecov]

Codecov Report

Merging #72 into dev will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##                dev      #72   +/-   ##
=========================================
  Coverage     89.76%   89.76%           
  Complexity       73       73           
=========================================
  Files            10       10           
  Lines           293      293           
  Branches          1        1           
=========================================
  Hits            263      263           
  Misses           29       29           
  Partials          1        1           

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