docs: add gitpod badge to `README.md`

[vil02]

This project is setup to be used in gitpod. However the gitpod badge is missing in README.md (other The Algorithms repos have such badge). This PR adds the missing badge.

[codecov[bot]]

Codecov Report

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

Comparison is base (9f71176) 97.60% compared to head (681fe37) 97.58%.

:exclamation: Current head 681fe37 differs from pull request most recent head 2aabbe6. Consider uploading reports for the commit 2aabbe6 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #192      +/-   ##
==========================================
- Coverage   97.60%   97.58%   -0.03%     
==========================================
  Files         142      123      -19     
  Lines        1796     1777      -19     
==========================================
- Hits         1753     1734      -19     
  Misses         43       43              

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