docs: avoid suggesting star dependencies.
docs: avoid suggesting star dependencies.

A `*` version constraint makes it hard/impossible to release a breaking
change (e.g. clap 2.0) without causing people's code to break.  `cargo
update` with a `*` dependency will happily move from (say) 1.4 to 2.0,
even though semver says these may be a breaking change. Using a more
precise dependency constraint gives the crate author more flexibility in
making breaking changes, and makes users of the crate happier if/when
they do happen.

https://github.com/rust-lang/rfcs/pull/1241 contains a bit more
discussion about this.

clap-rs/clap,kbknapp/clap-rs,cstorey/clap-rs,Vinatorul/clap-rs,kbknapp/clap-rs,Vinatorul/clap-rs,sru/clap-rs,clap-rs/clap,sru/clap-rs,sru/clap-rs,Vinatorul/clap-rs,cstorey/clap-rs,cstorey/clap-rs,clap-rs/clap