I live in central Texas and am building my first hugelkultur bed. There is a ton of limestone beneath this bed and I'm wondering what can be added to neutralize this somewhat. I dug down to the limestone and have added the logs and twigs. What next?

You shouldn't need to neutralize the limestone. The rotting of the wood and other debris will have an acidifying effect and over time it should be balanced enough. Once you've got the logs and twigs down continue on with the advice from the article: cover that with mulch, compost, soil, etc until you've got a nice big bed. Then plant.