message: fixed img on the main doc page
diff: Hyperledger Iroha documentation
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  -.. image:: https://github.com/hyperledger/iroha/raw/develop/docs/image_assets/iroha_logo.png
  +.. image:: ../image_assets/iroha_logo.png
  Welcome! Hyperledger Iroha is a simple blockchain platform you can use to make trusted, secure, and fast applications by bringing the power of permission-based blockchain with Byzantine fault-tolerant consensus. It's free, open-source, and works on Linux and Mac OS, with a variety of mobile and desktop libraries.
  