Add top level package file
Add top level package file

To prevent this warning:

$ go get -d github.com/hyperledger/burrow
can't load package: package github.com/hyperledger/burrow: no Go files in /home/vtp/go/src/github.com/hyperledger/burrow

Signed-off-by: Sean Young <ed09b1b8de87fefa3fbdeb9d60effff215445504@monax.io>

silasdavis/burrow,eris-ltd/eris-db,eris-ltd/eris-db,silasdavis/burrow,silasdavis/burrow