Write code coverage instructions to CONTRIBUTING.md
Write code coverage instructions to CONTRIBUTING.md

This should help everyone get quickly started on test code coverage measurement
using Tarpaulin.

Signed-off-by: Paul Mabileau <a3aedf8c87c1af3dfeae5f57ddf6d5ae82ad981f@hotmail.fr>

sunjay/turtle,sunjay/turtle,sunjay/turtle