Initializes the interface to the LCD screen, and specifies the dimensions (width and height) of the display. begin() needs to be called before any other LCD library commands.
lcd.begin(cols, rows, charsize)
lcd: a variable of type LiquidCrystal
cols: the number of columns that the display has
rows: the number of rows that the display has
charsize (optional): the number of dots the display has per character: LCD_5x8DOTS for 5x8, LCD_5x10DOTS for 5x10. (default: LCD_5x8DOTS)