configuration to use pomicons in README

This is the full explanation with screenshot on how to have the pomodoro symbols

After merge you need to change the screenshot's url https://github.com/gabrielelana/vim-airline-tomato/raw/pomicons-configuration/.screenshot/pomicons.png in something like https://github.com/Zuckonit/vim-airline-tomato/raw/master/.screenshot/pomicons.png

Merge it if you like it :smile: