(DOCSP-17931): Added Atlas connection string note.

Author: corryroot

source/reference/connection-string.txt

@@ -19,6 +19,14 @@ between applications and MongoDB instances in the official MongoDB
 :driver:`Drivers </>`. For a list of drivers and links to
 driver documentation, see :driver:`Drivers </>`.
 
+.. note::
+
+   The MongoDB |service| UI provides the connection strings for you 
+   when you :atlas:`connect to a database deployment 
+   </connect-to-database-deployment/>`. To register a new |service| 
+   account, start at the :mdbacct:`MongoDB Atlas registration page 
+   </register?tck=docs_atlas>`.
+
 Connection String Formats
 -------------------------