README update

[sigmafelix]

• Detailed description on the parallel backend initialization
• Flowchart for helping users to choose a proper par_* function

[codecov[bot]]

Codecov Report

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

Project coverage is 99.70%. Comparing base (20d3be2) to head (3b4c0f2). Report is 7 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #62 +/- ## ======================================= Coverage 99.70% 99.70% ======================================= Files 5 5 Lines 676 676 ======================================= Hits 674 674 Misses 2 2 ```

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