extending string

["Aljaz" <aljaz.fajmut@...>]

Hello

I'm new to lua and I have read documentation.
However, I'm having difficulties finding out how to 'extend' lua string this 
way:
str = 'some string... '
str += 'and some added text'

str would be: 'some string... and some added text'.

Basiclly I want to just add (asign) data to the end of string.
How to do this?

Thanks for help