Update DataPersistenceAndCaching.md

Example of caching usage in MongoDB Atlas

microsite/docs/ConfigurationOptions/DataPersistenceAndCaching.md

@@ -62,6 +62,12 @@ MONGODB_CREDENTIALS=/path-to-credentials.pem
 - The MONGODB_URI variable should be set to the connection URI of your MongoDB instance. If using Atlas, you should find this option in the Connect section of your Atlas cluster. For a local instance, the default string is usually `mongodb://localhost:27017` while the service is running.
 - The MONGODB_CREDENTIALS variable is optionally used when configuring MongoDB Atlas. CCF will need the credentials to be stored and referenced in order to connect.
 
+Example of usage in MongoDB Atlas:
+``` .env
+CACHE_MODE=MONGODB
+MONGODB_URI=mongodb+srv://user:password@cluster-uri
+```
+
 After calculating estimates for the first time, the app will create a new collection titled “ccf”. Estimates will be separated into timestamps and stored into a collection that is named according to the current grouping method (i.e. “estimates-by-month).
 
 ### Paginating Estimates