Clarify recco to install as a soft dependency
Clarify recco to install as a soft dependency

On IRC it was mentioned that clippy is not meant to be installed as a hard dependency.
As it was, the README placed the hard dependency instructions first and did not
mention the recommendation, misleading users into making it a hard dependency. A
quick survey of the dependent crates on crates.io reveals the reach of this issue.
graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust