Troubleshoot: Run the building tool with only one user

[FabioPinheiro]

There's no reason to run the building tool (sbt) with super user permissions.

Running the build with sudo (root user) or any other user will generate the target folder ower by that user.
Then if you run with another user this can lead to unintuitive permission errors. Like #216

Let's add to the troubleshooting section in README with some of those errors.
Also, add a way to fix it. By doing a clean git clone ..., or run sbt clean or even by changing every single file on the folder chown -R USER:GROUP atala-prism-mediator