Use OP_RETURN to store merkle root in bitcoin blockchain

Instead of sending to a bitcoin address, which adds to the set of unspent outputs (UXTO), you should use OP_RETURN. More info: https://en.bitcoin.it/wiki/Script#Provably_Unspendable.2FPrunable_Outputs

Many bitcoin clients index the UXTO set and if used to store data in an automatic way, the set increases faster. A better way is to use OP_RETURN and let those transactions be pruned from the set. The OP_RETURN can be followed by arbitrary data and you can store the merkle root. It is the same principles the [Proof of Existence](http://www.proofofexistence.com/about) site is using.

Start with the issue's proposal and the linked Bitcoin Script and Proof of Existence references. Identify the repository entry points for storing the merkle root, then confirm the intended OP_RETURN behavior and define completion criteria with maintainers before making changes.

Written by the indexing model from the issue text.