docs: Add LGTM badge

[SvanBoxel]

This will add a banner containing our LGTM status: https://lgtm.com/projects/g/SvanBoxel/delete-merged-branch/

[codecov]

Codecov Report

Merging #157 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #157   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           3      3           
  Lines          29     29           
  Branches        4      4           
=====================================
  Hits           29     29

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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities (and 0 Security Hotspots to review)
0 Code Smells

No Coverage information
No Duplication information