Add note about redirect_uri

README.md

@@ -175,12 +175,15 @@ vim config_init.sh
 
 This script will automatically create and add a new client in the Oauth server, returning a client id and a client secret. You need to keep these two token to configure Mattermost. Please be sure the client secret remained secret.
 
-The redirect uri in the script must comply with the hostname of your
-Mattermost server, or else Mattermost will not be able to get data from
-the Oauth server. If you update your hostname, you will need to update
-this value.  Here is an example query:
+The redirect uri in the script must comply with the hostname of your Mattermost server, or else Mattermost will not be able to get data from the Oauth server. If you update your hostname, you will need to update this value.  Here is an example query:
 
-```UPDATE oauth_clients SET redirect_uri = 'https://example.com/signup/gitlab/complete' WHERE client_id = '1234567890';```
+```sql
+UPDATE oauth_clients SET redirect_uri = 'https://mattermost.company.com/signup/gitlab/complete' WHERE client_id = '1234567890';
+```
+
+**Warning** : The `redirect_uri` parameter should be strictly the same as the one given by Mattermost to Oauth server during authentication. If your Mattermost server uses HTTPS, make sure the `redirect_uri` begin with `https`.
+
+*Note* : Mattermost build the `redirect_url` from the parameter `SiteURL` in `config.json`. Thus, if you set this parameter to `https://mattermost.company.com`, Mattermost will use the following redirect URL : http**s**://mattermost.company.com/signup/gitlab/complete (`SiteURL` + '/signup/gitlab/complete').
 
 ## Configuration