Decoration can be easily inserted in ABC notation using a set of characters.
Here’s a list of the most common decorations:

~    a roll, used mainly in Irish and Scottish music.   
.    *staccato* mark.  
H    *fermata* mark.  
L    accent or emphais. 
M    lowermordent
P    uppermordent

Other decorations use the ! syntax, which is more explicit:


Here’s a simple sequence with some decoration thrown in:

X: 1
C D !trill!E2 | !mordent!D E F G | ~G3

For a full list of all available decorations, visit the official ABC notation reference website