Document --force

[FokkeZB]

Summary

Documents install/add --force, which does what #858 asks for.

Test plan

Run yarn add --help or yarn install --help and --force should be documented as refetch all packages

[sebmck]

--force skips our integrity cache and forces execution of install scripts so it does a few more things. I think it'd be more accurate to summarise the description as "ignore all caches"

[FokkeZB]

@kittens I changed it, although I must say I find the option name itself not very descriptive. Force what? It's also confusing that you have --offline, --prefer-offline and then to get the opposite you need --force.