Update readme with software majurity badge

[Ozaq]

Description

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

🌦️ >> Documentation << 🌦️
https://sites.ecmwf.int/docs/dev-section/eckit/pull-requests/PR-259

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.27%. Comparing base (95cf4fa) to head (6e09804).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #259      +/-   ##
===========================================
+ Coverage    66.25%   66.27%   +0.01%     
===========================================
  Files         1130     1130              
  Lines        57931    57931              
  Branches      4390     4390              
===========================================
+ Hits         38380    38391      +11     
+ Misses       19551    19540      -11     

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