Added coverage details on contribution guidelines

[tlunet]

Following #262

[codecov]

Codecov Report

Base: 70.61% // Head: 70.59% // Decreases project coverage by -0.03% ⚠️

Coverage data is based on head (d712992) compared to base (07db8c0).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #265      +/-   ##
==========================================
- Coverage   70.61%   70.59%   -0.03%     
==========================================
  Files         229      229              
  Lines       19646    19646              
==========================================
- Hits        13874    13869       -5     
- Misses       5772     5777       +5     

Impacted Files	Coverage Δ
...C/projects/soft_failure/implicit_sweeper_faults.py	94.44% <0.00%> (-2.09%)	⬇️
...mentations/problem_classes/Van_der_Pol_implicit.py	96.72% <0.00%> (-1.64%)	⬇️
pySDC/projects/soft_failure/generate_statistics.py	74.87% <0.00%> (-0.51%)	⬇️

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[pancetta]

In case you wonder why this had an effect on coverage: there's one project which uses 1000s of tests to generate statistics. Looks like this does not have a random seed, so every once in a while the tests behave slightly differently and a few lines are not visited.

[tlunet]

Looks like fun 😆 ... I was wondering indeed, thanks for the explanations