message: Add intro blurb about the public testnet.
diff: @@ -10,6 +10,8 @@ This code is currently alpha-quality and under rapid development. That said,
  there is plenty early experimenters can do including running a private multi-node test network and
  developing applications (smart contracts).
  +The public testnet described in the [wiki](https://github.com/EOSIO/eos/wiki/Testnet%3A%20Public) is running the `dawn-2.x` branch. The `master` branch is no longer compatible with the public testnet. Instructions are provided below for building either option.
  +
  # Resources
  1. [EOS.IO Website](https://eos.io)
  2. [Documentation](https://eosio.github.io/eos/)
  