Update README.md information with more details

[metodiew]

We should update the readme file with adding more details about DX Starter theme, popular helper functions, folder structure, etc

[xavortm]

From #30 by @mariyan-dimitrov

I was following the instructions in the README file and in section Updating installed Node.js is trough NPM: I encountered a command (sudo npm cache clean -f) which I need to run in my console.
After doing that, I received an error with the following message:
/usr/bin/env: ‘node’: No such file or directory
Also, there is a typo in the naming of the section.