Added explanation of how to add ~/bin to path to install script

[Cali0707]

Fixes #16

With this new code, running the install script without ~/bin in my path produces the following message:

./install.sh      
Your path is missing ~/bin, you might want to add it.
If you use bash, add 'export PATH="${PATH}:${HOME}/bin"' to your .bashrc file
If you use zsh, add 'export PATH="${PATH}:${HOME}/bin"' to your .zshrc file
If you use fish, add 'fish_add_path -m ~/bin' to your config.fish file

[Leo6Leo]

LGTM and APPROVED