Update README.md (#2506)

We've added a Salesforce port of this library written in Apex and adapted to Salesforce governor limits.

README.md

@@ -229,6 +229,7 @@ maintenance processes.
 *   Clojure: https://github.com/randomseed-io/phone-number
 *   R: https://github.com/socialresearchcentre/dialr/
 *   Elixir: https://github.com/socialpaymentsbv/ex_phone_number
+*   Salesforce: https://appexchange.salesforce.com/appxListingDetail?listingId=a0N3A00000G12oJUAR
 
 Alternatives to our own versions: