Instruct to install in editable mode in CONTRIBUTING docs

[AdrianDAlessandro]

Description

Package should be installed in editable mode for tests to pass. This is already done in CI and was in the old (currently published) version of the docs, but not the most up to date version of the docs.

Fixes #330

Type of change

• Documentation (non-breaking change that adds or improves the documentation)
• New feature (non-breaking change which adds functionality)
• Optimization (non-breaking, back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (whatever its nature)

Key checklist

• All tests pass (eg. python -m pytest)
• The documentation builds and looks OK (eg. python -m sphinx -b html docs docs/build)
• Pre-commit hooks run successfully (eg. pre-commit run --all-files)

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added or an issue has been opened to tackle that in the future. (Indicate issue here: # (issue))

[barneydobson]

lgtm!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.76%. Comparing base (1ec99d4) to head (ef300b4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #331   +/-   ##
=======================================
  Coverage   86.76%   86.76%           
=======================================
  Files          22       22           
  Lines        2199     2199           
  Branches      280      280           
=======================================
  Hits         1908     1908           
  Misses        214      214           
  Partials       77       77           

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