Troubleshoot: Run the building tool with only one user

[FabioPinheiro]

Background

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 owned by that user.
Then if you run with another user this can lead to unintuitive permission errors. Like #216

DoD

• The troubleshoot SHOULD help the user
  • know that he should not run as a super user (giving an example of the no intuitive errors)
  • solve the original problem mention on java.io.IOException while building an image of the mediator #216. User clone as repo as user A and is run as User B (without permissions on the folders)

Implementation Details/Hints

• Add to the troubleshooting section in README.md with some of those errors.
• Mentioned that the user should not run SPT as a super user
• Add a way to fix it java.io.IOException while building an image of the mediator #216. 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

[yshyn-iohk]

@FabioPinheiro, is this issue still relevant?

[FabioPinheiro]

It's still relevant. If anyone wants to pick this up is a easy way to add their name to the contributor list.
Just need to go to the section Troubleshoot and add the error message and the commands to fix it.
As described on two tickets #216 #217

[pparker]

@FabioPinheiro to check if this is still valid

[FabioPinheiro]

I refined the ticket and put it as ready for developing.
It's a simple update in the README anyone can do it quickly.