Add zsh instead of bash as an alternative in installation messages

As in the topic, I think it'll be reasonable to add note about adding both to `.bashrc` and `.zshrc` the following snippet:
```
export PATH="$HOME/.pyenv/bin:$PATH"
eval "$(pyenv init --path)"
eval "$(pyenv virtualenv-init -)"
```

Personally, I wasted a lot of time searching for an error while installing Python via `pyenv` because of not adding this to `.zshrc.