update README file

[code4happylife]

Basically, the users needs to execute source $HOME/.bashrc or source $HOME/.zshrc after they have followed the

installation instruction:

NOTES
   Installation:
       Put something like this in your $HOME/.bashrc or $HOME/.zshrc:

              . /path/to/z.sh

So, to make the installation procedure more straightforward, I tried to add this notes to the README file.

[pabloab]

I would just clarify the change will take effect on next opened terminal. What I 'd change on the Readme is the use of the more readable alternative to the dot, source.