Add info to add `eval ruby.env` into `.bashrc`

[matrdr]

In order to execute commands installed by gems, one has to add the line

eval `ruby.env`

to one's .bashrc. This is only stated in this old article but not in the package description or anywhere else. In my opinion this should be more clearly stated as I think this is a useful feature and (not entirely sure) the default behaviour if I were to install it using apt.