KohaRest: Add a comment about required configuration in Koha. (#1656)

config/vufind/KohaRest.ini

 [Catalog]
 ; The API address without any version such as v1
 host = "http://koha-server/api"
+; N.B. Make sure the RestOAuth2ClientCredentials preference is enabled in Koha, and
+; that the Koha server has the Net::OAuth2::AuthorizationServer cpan module
+; installed (minimum version 0.16).
+; See https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=20624 for more
+; information.
 ; You should create a Koha user for the API with at least the following privileges:
 ; - circulate_remaining_permissions
 ; - catalogue