added a user property method example to the readme

Author: Ata Kuyumcu

README.md

@@ -57,6 +57,7 @@ int main(int argc, char *argv[]) {
 	Countly& ct = Countly::getInstance();
 	// OS, OS_version, device, resolution, carrier, app_version);
 	ct.SetMetrics("Windows 10", "10.22", "Mac", "800x600", "Carrier", "1.0");
+	ct.SetCustomUserDetails({{"Account Type", "Basic"}, {"Employer", "Company4"}});
 	// Server and port
 	ct.Start("abf2034f975393fa994d1cf8adf9a93e4a29ac29", "https://myserver.com", 403);
 	ct.SetMaxEventsPerMessage(40);