Migration command improvement

[pierrelalanne]

Context

Having a deprecated config triggers an error message : Run ggshield config migrate to migrate it to the latest version.. However, running this command can in turn trigger an error if it is not launched from the correct location.

The issue

We should make it clearer both in the README.md and in the deprecation message that the command needs to run from the path of the .gitguardian.yml file.

Extra

This would be great if we can make the command more robust in general.