Tiny improvement to contributing guide

[alessandrofelder]

Description

What is this PR

• [ ] Bug fix
• [ ] Addition of a new feature
• [x] Other

What does this PR do?

• fixes a link in the readme
• adds a notice to contributors that there are local tests.

References

Closes #253

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (a5af51c) 86.39% compared to head (cc53b70) 86.39%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## mega-fix #284 +/- ## ========================================= Coverage 86.39% 86.39% ========================================= Files 26 26 Lines 1205 1205 ========================================= Hits 1041 1041 Misses 164 164 ```

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