How do i transform this dataframe
Category  Comments
A          hi, no
B          hello 
C          good. morning
into:
Category Comments
A          hi
A          no
B          hello
C          good
C          morning
How do i transform this dataframe
Category  Comments
A          hi, no
B          hello 
C          good. morning
into:
Category Comments
A          hi
A          no
B          hello
C          good
C          morning