Simple procedural texture. You may need to adjust the scale on the noise node and/or the factor of the multiply node connecting it to the displacement output, this will be dependent on the scale of your target object. The latter can also be used to affect how smooth or how bumpy you want your rind to be. By default, its scaling and such is calibrated to make the test object provided by Blendermada look like an orange. By manipulating these factors and also the color ramp node, you could make other kinds of citrus fairly easily, for example lemons, key limes, etc.