Add a link to the account page.

Which is not obvious, I searched in admin.gandi.net for a bunch of
minutes before finding account.gandi.net.

Also v4/v5 being XMLRPC/REST split is not obvious.

gandi/cli/commands/root.py

@@ -16,6 +16,13 @@ def setup(gandi):
     """
     intro = """Welcome to GandiCLI, let's configure a few things before we \
 start.
+
+If you don't already have them, you can generate API keys from
+your Gandi account:
+
+=> https://account.gandi.net/ for the v5 (REST API)
+=> https://v4.gandi.net for the v4 (XMLRPC API)
+
 """
 
     outro = """

gandi/cli/modules/dns.py

@@ -20,6 +20,7 @@ class Dns(GandiModule):
 
     """
 
+    # API documentation: https://doc.livedns.gandi.net/
     api_url = 'https://dns.api.gandi.net/api/v5'
 
     @classmethod