Add social media badges to README

[IgorTatarnikov]

Following up on brainglobe/cellfinder#563

Changes include:

• Adding a badge linking to the image.sc forum (shamelessly stolen from https://github.com/napari)
• Adding badges for bluesky and mastodon

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.60%. Comparing base (49d971a) to head (18d26fa).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #435   +/-   ##
=======================================
  Coverage   86.60%   86.60%           
=======================================
  Files          27       27           
  Lines        1239     1239           
=======================================
  Hits         1073     1073           
  Misses        166      166           

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[sfmig]

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