Document the new method to get a single user in readme file.

Author: Muhammad Anas

README.md

@@ -50,6 +50,9 @@ $client->users->create([
 // Delete a user by ID
 $client->users->deleteUser("570680a8a1bcbca8a90001b9");
 
+// Get a user by ID
+$client->users->getUser("570680a8a1bcbca8a90001b9");
+
 // Add companies to a user
 $client->users->create([
   "email" => "test@intercom.io",