docs: Explain the provider field for git credentials configuration

docs/git.md

@@ -57,6 +57,14 @@ This will configure these options for one repository.
 
 To enable for all OpenNeuro repositories add this to your [git configuration file](https://git-scm.com/docs/git-config#FILES).
 
+```cfg
+[credential "https://openneuro.org"]
+  useHttpPath = true
+  helper = "/path/to/openneuro git-credential"
+```
+
+If you are using [Git Credential Manager](https://github.com/git-ecosystem/git-credential-manager) add the provider entry to avoid duplicating entries.
+
 ```cfg
 [credential "https://openneuro.org"]
   useHttpPath = true