Added more arguments details
Added more arguments details

Just noticed that the Readme.md wasn’t updated to include the latest functions of the steamgrid program.
So I’ve added those.

However : 
— The instructions might be too over explicit or over-complicated, despite being extremely fluent in English, I tend to express myself in those manners, so this might need to be rephrased.
— I didn’t add all the available instructions from `steamgrid -h`, including this exact help argument. This can be added, of course, but you might prefer to not precise it, to keep the Readme.md description as concise as possible.
boppreh/steamgrid