Update README on shell-specific installation

Some users do not override their default system shell. In that case rbenv's shell auto-detection fails. To help user to fix this problem, an information on manual shell selection should be provided in docs. Otherwise the only way to solve a problem is to dig into `rbenv-init` source.

Steps to reproduce:

~~~ sh
$ echo $SHELL
/bin/bash
$ zsh
$ echo $SHELL
/bin/bash
$ eval "`rbenv init -`"
/usr/lib/rbenv/libexec/../completions/rbenv.bash:16: command not found: complete
~~~