You just need to add your title under "THE TITLE"...
var legend = L.control({position: 'topleft'});
legend.onAdd = function (map) {
var div = L.DomUtil.create('div', 'info legend'),
grades = [50, 100, 150, 200, 250, 300],
labels = ['<strong> THE TITLE </strong>'],
from, to;
for (var i = 0; i < grades.length; i++) {
from = grades [i];
to = grades[i+1]-1;
labels.push(
'<i style="background:' + getColor(from + 1) + '"></i> ' +
from + (to ? '–' + to : '+'));
}
div.innerHTML = labels.join('<br>');
return div;
};