Updated Readme.md to make quality parameter better documented.

[Kiricon]

Added example of numbers that can be used for quality parameter and explained the current limit of numbers above the 83% quality threshold.

"equivalent to libjpeg quality" does not have inherent meaning to users without the context of the documentation.

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[Kiricon]

I signed it!

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[kornelski]

"require modifying the code to go bellow the set threshold." - that can easily be interpreted as "require hacking the argument check to allow lower number" rather than "rewriting the compressor to tune it to lower values".

The check isn't there to annoy people and make them compile source with a lower number. It's there to stop people from wasting their time on using quality that this compressor isn't good at.

[Kiricon]

@pornel That's a very great point I had not thought of. Is there by chance a way we can change that sentence so that we can still give a user heads up about the limit, and inform them that the compressor should not be bellow that threshold for the reasons you outlined? Or Perhaps we should remove that section from the read me and leave in the example "(0 - 100)"?

[kornelski]

Perhaps something like this:

You can pass a --quality Q parameter to set quality in units equivalent to libjpeg quality (84 - 100). Guetzli works best with high qualities. 84 is the lowest supported quality.

[Kiricon]

@pornel Thanks for the example. I think that the wording is much more elegantly in these terms.

I updated my pull request to include the text in your example.

Thanks again for the feedback!