docs(readme): add --rm to Docker example

[glensc]

there's no need to keep the used container around.

ideally one could persist the used data paths, i.e profile data.

-v $(pwd)/state/config:/root/.config -v $(pwd)/state/cache:/root/.cache

but I haven't investigated so far what paths need to persist between runs.

[AlexanderMaxRanabel]

good idea

[fathyb]

Thanks!