docs: update README.md

[muktihari]

• Remove OpenSSF Best Practices badge as it can be viewed from the OpenSSF Scorecard badge's link.
• Update Motivation description and fitactivity description

[codecov-commenter]

Codecov Report

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

Project coverage is 100.00%. Comparing base (20cc308) to head (6ad5388).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #410 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 41 41 Lines 3693 3693 ========================================= Hits 3693 3693 ```

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