[README.md] better info about parameters

[pawelmhm]

added better example of paramters to docs + link to page with all parameters @edorofeev

[codecov]

Codecov Report

Merging #16 (48c0f5f) into master (8cefd24) will not change coverage.
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[pawelmhm]

@Gallaecio one thing to note is that some parameters are not documented in the website of Fetch API. I'm not confident if they are official or not? @edorofeev Should we also add parameters to website? I can do it too, it will speed up things next time I need to set up some spider for Fetch api

[elacuesta]

Approved, not merging yet because of the open questions at #16 (comment)