README: add marketplace as installation option

[strangelookingnerd]

Since the marketplace does provide a convenient method to install plugins into a running IDE as well as downloading the latest version it should be mentioned in the README.

On a side-note: I was tempted to remove the "Manually" option that references the GitHub Releases. I would think that it may become an issue in case a plugin release was pushed to GitHub but is still awaiting verification for the marketplace. This does not only create a mismatch between those two sources but may also impose further issues in case the verification is not succesful and the version is being revoked. Idealy the README would only propose the "prefered" or "official" ways of obtaining a plugin - which would be the marketplace I guess.

[codecov-commenter]

Codecov Report

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

Project coverage is 26.92%. Comparing base (ae35708) to head (6afc257). Report is 30 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #463      +/-   ##
==========================================
- Coverage   27.58%   26.92%   -0.67%     
==========================================
  Files           4        4              
  Lines          29       26       -3     
==========================================
- Hits            8        7       -1     
+ Misses         21       19       -2     

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[hsz]

Thanks!