docs(core): add NGImap4AuthMechanism = "plain"; to sample config file

forcing `AUTHENTICATE` using Add NGImap4AuthMechanism = "plain"; is necessary
for setups where users have UTF-8 characters in their password

Scripts/sogo.conf

@@ -32,6 +32,7 @@
   //SOGoMailingMechanism = smtp;
   //SOGoForceExternalLoginWithEmail = NO;
   //SOGoMailSpoolPath = /var/spool/sogo;
+  //NGImap4AuthMechanism = "plain";
   //NGImap4ConnectionStringSeparator = "/";
 
   /* Notifications */