docs(twilio-run): adds docs for ngrok config options

packages/twilio-run/README.md

@@ -159,6 +159,28 @@ twilio-run --ngrok=subdomain
 twilio-run --ngrok --ngrok-config=./ngrok.yml --ngrok-name=example
 ```
 
+#### ngrok
+
+`twilio-run` lets you open a tunnel using [ngrok](https://ngrok.com/) (see the examples above). By default, just setting the `--ngrok` flag will use a randomly generated subdomain. If you have a paid for ngrok account, you can customise this experience.
+
+##### Custom ngrok subdomain
+
+Run the following command:
+
+```
+twilio-run --ngrok=subdomain
+```
+
+This will start the `twilio-run` server and also tunnel to it over ngrok via the domain `subdomain.ngrok.io`.
+
+##### Custom ngrok config
+
+You can create an [ngrok config file](https://ngrok.com/docs#config-location) which allows you to name tunnels and include other settings for those named tunnels, such as the subdomain, auth, or host headers.
+
+You can choose to run a named tunnel with `twilio-run` by passing the `--ngrok-name` flag. This will find the named tunnel in your default config, over-ride the `proto` to http and the `addr` to the port your Twilio Functions are running on, otherwise keeping the rest of the settings.
+
+You can also choose to use a different config file by setting the `--ngrok-config` flag.
+
 ### `twilio-run deploy`
 
 Deploys your project to Twilio. It will read dependencies automatically from your `package.json`'s `dependencies` field and install them. It will also upload and set the variables that are specified in your `.env` file. You can point it against a different `.env` file via command-line flags.