Update README.md

[michaelknoch]

Type of change: docs

Motivation (current vs expected behavior)

• refer swift 5.1 badge
• remove swift package manager from readme, reference cmake and ninja

Please check if the PR fulfills these requirements

• [ ] Self-review: I am confident this is the simplest and clearest way to achieve the expected behaviour
• [ ] There are no dependencies on other PRs or I have linked dependencies through Zenhub
• [ ] The commit messages are clean and understandable
• [ ] Tests for the changes have been added (for bug fixes / features)

[codecov[bot]]

Codecov Report

Merging #331 (b6eed37) into master (31b422b) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #331   +/-   ##
=======================================
  Coverage   51.25%   51.25%           
=======================================
  Files          87       87           
  Lines        2899     2899           
=======================================
  Hits         1486     1486           
  Misses       1413     1413           

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Continue to review full report at Codecov.

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