Add note to readme about lack of class level variables.
Add note to readme about lack of class level variables.

kovidgoyal/rapydscript-ng,kovidgoyal/RapydScript,atsepkov/RapydScript,atsepkov/RapydScript,atsepkov/RapydScript,kovidgoyal/rapydscript-ng,kovidgoyal/RapydScript,kovidgoyal/rapydscript-ng,kovidgoyal/rapydscript-ng,kovidgoyal/RapydScript