BlockchainWriter README

[lautarodragan]

Add a README.md to the BlockchainWriter module.

See https://github.com/poetapp/node/pull/145#issuecomment-397540474 for an example, reasoning and discussion.

This issue can be tackled on its own or as part of any PR that introduces changes to the BlockchainWriter module.

[kennylavender]

@lautarodragan is there anything else that we would like to add to this README before closing this?

https://github.com/poetapp/node/tree/master/src/BatchWriter

[kennylavender]

nvm ignore that, I miss read, this is for blockchain, not batch.