Text outlines were added (setOutlineThickness & setOutlineColor) in SFML 2.4 exactly 6 years ago, any reason you don't want to use a more recent version of SFML?
Anyway, if you can't update to 2.5, you can make an outline by drawing the same text 8 times around your text with 1 (or more) pixel offset in each direction, or create an outline in a fragment shader if you know some GLSL.