docs: add install instructions to contibution guide

[mbtools]

Adds instruction on how to install the projects avoiding issues related to dependency conflicts (see #2123). This is a trap esp. new contributors fall into.

PS: Once the issue is fixed, the command be changed to the usual npm install, of course.

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (where applicable)
• Tests written (where applicable)
• References provided in PR (where applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (4af6124) to head (e91d3fb).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2621   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          114       114           
  Lines         2562      2562           
  Branches       649       649           
=========================================
  Hits          2562      2562           

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