doc: Explain how to update chainTxData in release process
doc: Explain how to update chainTxData in release process

Adds a short explanation how to update chainTxData to the release
process. Mention where to get the data, and link to an example.

chaincoin/chaincoin,chaincoin/chaincoin,chaincoin/chaincoin,chaincoin/chaincoin,chaincoin/chaincoin,chaincoin/chaincoin