Add full URL to Contribution Guidelines

[zkoppert]

Pull Request Checklist

Please review the Contribution Guidelines before submitting.

• Pulling from a topic/feature/bugfix branch (right side).
• Pulling against the canary branch (left side).
• Code passes linting.
• Code passes unit tests.
• Code coverage is the same or better.
• You have previously submitted a Contributor License Agreement or have contacted a maintainer to request one.

Description

The link is broken when executing it from a pull request unless the full URL is given.

[codecov]

Codecov Report

Merging #12 into canary will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           canary      #12   +/-   ##
=======================================
  Coverage   78.93%   78.93%           
=======================================
  Files          13       13           
  Lines         413      413           
=======================================
  Hits          326      326           
  Misses         87       87