Add a sample app that sends a Twilio SMS anytime Mongo is updated

This was an interesting example I found in the Mulesoft GitHub repos.  It seemed like
a perfect candidate for Riff/Mu/Lambda, so I sketched out what it might look like.

examples/mongo_twilio/README.md

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+# mongo_twilio
+
+This sample sends an SMS via Twilio to any new customer entry added to a MongoDB database.
+
+This is derived from Mulesoft's sample app here:
+https://github.com/mulesoft/twilio-connector/tree/master/example-send-sms-interact-with-mongo
+
+Assuming we are deploying to AWS, running `$ mu deploy app.js` creates a new CloudFormation stack.  That stack includes
+a single lambda that is triggered off modifications to the `clients` MongoDB database.  This database is not provisioned
+by this stack; instead, we use name-based discovery to find an existing database configured in the environment.
+
+In fact, many configuration entries are automatically plumbed around in this example, which are passed manually in the
+Mulesoft example.  For instance, the Twilio account SID and "from" phone number, are configured per stack deployment.
+

examples/mongo_twilio/app.js

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+// This code is the moral equivalent of Mulesoft's example "code" here:
+// https://github.com/mulesoft/twilio-connector/blob/master/example-send-sms-interact-with-mongo/src/main/app/mule-config.xml
+
+var mu = require("mu");
+var mongo = require("mu-mongo");
+var twilio = require("mu-twilio");
+
+var clients = mongo.connect("clients");
+clients.forEach(client =>
+    twilio.sendSmsMessage({
+        to: cust.phone,
+        body: `Hi ${client.name}! Your account balance is USD${client.accountBalance}`,
+    });
+);
+