Update README to initialize instrumentator after app startup

[mdczaplicki]

No description provided.

[trallnag]

What is the advantage of doing it like this?

[mdczaplicki]

The advantage is that it works only like this.
With regards to this #80

[codecov]

Codecov Report

Merging #168 (44381aa) into master (3f1c500) will decrease coverage by 0.31%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #168      +/-   ##
==========================================
- Coverage   93.48%   93.17%   -0.32%     
==========================================
  Files           4        4              
  Lines         261      249      -12     
==========================================
- Hits          244      232      -12     
  Misses         17       17              

Impacted Files	Coverage Δ
...ometheus_fastapi_instrumentator/instrumentation.py	90.90% <0.00%> (-0.32%)	⬇️
src/prometheus_fastapi_instrumentator/metrics.py	97.29% <0.00%> (-0.23%)	⬇️

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[trallnag]

Thanks