[FEATURE] Update README template for repository creations

[GuillaumeFalourd]

What would you like to be added:

• Update the README template for repository creations with a explanation about how to update a formula.

Why is this needed:

• Today, it explains how to fork the repository and add a formula, but it doesn't explain how update a formula from the repository and test it.

To do so, it has to clone the repo, and add it as a workspace with a higher priority than the imported repository with rit add repo command. Then, update the formula and test it with the associated command.