To add another notch in your belt is simply a way of keeping some type of count. For a braggart or Lothario, it might be the number of sexual partners, for a gunslinger in the the old American West, it might be number of men you've killed, it might also be the number of mountains you've climbed.
In general, it's no longer a physical notch in a physical belt (or gun stock, or bed post), it's method of tracking something that you care about. In this specific case, it's something that the City of Houston will be proud of, adding another accomplishment, a Space Port, to it's list of commercial attractions.