docs: updated readme with third party plugins permissions

[carloeusebi]

Updated readme for #547.

[what-the-diff]

PR Summary

• Addition of "Third-Party Plugins Permissions" section to instructions manual
  This update includes a new section in the README document. This section, specifically titled "Third-Party Plugins Permissions", serves to guide users on how to manage permissions associated with third-party plugins in our software.

• Guidance on specifying permissions using the configuration file
  The new content also provides clear instructions on how these privileges can be specified using a file named config. This indication allows users to configure the system according to their needs without extra IT help.

• Introduction of an example on setting permission prefixes
  To further aid our users' understanding, an example demonstrating how they can set up 'permission prefixes' for different elements like resources, pages and widgets has been added. This example can serve as a template for users to base their configurations on, making the process more manageable and less daunting.