Fix usage in README.md

[benoit74]

Usage section of README.md is quite confusing:

• it mention that it is possible to run warc2zim --help while this is both not working (you need to be inside the venv) + mostly useless (very few users will come with an existing WARC file)
• the shm-size parameter is not needed anymore, browsertrix crawler uses by default the new --disable-dev-shm-usage
• it mention many parameters (shm-size, workers, waitUntil) which are not strictly needing, adding more confusion than necessary
• it mention the --scroll parameter which does not exists
• probably other stuff to fix as well

[rgaudin]

it mention that it is possible to run warc2zim --help while this is both not working (you need to be inside the venv) + mostly useless (very few users will come with an existing WARC file)

To me this is a regression. zimit image is very convenient way to use warc2zim for WARC users. I think we should keep the feature (and it's as cheap as a symlink!)