Tell the user that Cargo.toml should be found at the root

[maelvls]

This is a follow-up to the comments in #157
(#157 (comment))

I changed the message

Cargo.toml must be in the workspace in order to support all features

to

A Cargo.toml file must be at the root of the workspace in order to support all features

so that we are reminded that the Cargo.toml should be at the root

[nrc]

Thanks!