Docs/readmes: add info about minimumCoverageThresholds

[alexandrehtrb]

I was looking for this feature but couldn't find it in README nor the main website; only found in the wiki.

It would be good to document it in:

• ReportGenerator readme
• GitHub Action readme
• Main website -> usage

[danielpalme]

Documenting and maintaining all settings in all those locations could become tedious.

Instead I added the following text to all the locations you suggested:

The more advanced settings are documented in the wiki.

Thanks for your hint!

[alexandrehtrb]

It would be good to at least add this info in the GitHub Action readme, because people use code coverage to fail CI/CD when below the thresholds... irongut/CodeCoverageSummary for example