Clarify the diesel_cli feature installation process
Clarify the diesel_cli feature installation process

For the naive reader, the previous phrasing can imply "--features mysql" actually installs the necessary mysqlclient library.   This new phrasing clarifies that the user can either A) install it on their own or B) exclude it from use by diesel_cli.

This came up in https://github.com/diesel-rs/diesel/issues/2214 where I mistakenly assumed the naive interpretation.
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