Add Stellar blockchain documentation

[plttn]

per: https://keybase.io/docs/server_security/merkle_root_in_stellar_blockchain

Unclear how you all build it, so I kept the name/number the same to be fixed against whatever build system you use. I kept the content the same, so it references the building against the bitcoin blockchain, which is a dead link on keybase.io/docs, but exists as the outdated version linked here)