Use the $a syntax and sed:
sed -i '$a some text' filename
-i parameter means in place. To test without changing the original file, remove the -i option. The results will be print on the screen.
A previous post explained how to insert a line anywhere but for completeness. Indeed, with the $a syntax you append content and therefore you can append text after a line. Therefore you cannot append text before the first line. Instead use the i command:
sed -i '1i some text on first line' filename