Updated docs for notification system

docs/messaging.md

-# VIPSLogic messaging
+# VIPSLogic push notifications ("messaging")
+VIPSLogic allows for push notifications for a few events:
+* Publication of a news message tagged with a crop category and a message type selected by the user
+* Publication of a field observation tagged with a crop category selected by the user
+* A pest prediction forecast for a given crop category at a specified location changes from low to high risk (may happen during the daily update of the forecast)
+
+**The notification system is generic; it can be adapted to send other types of notifications.**
+
+The notifications may be sent as emails or tests (SMS), depending on the VIPSLogic server's available message gateways. See how to configure this below.
+
+The user can change their notification preferences on their personal settings page (see illustration below)
+
+![User preferences for notifications](./illustrations/messaging_user_preferences.png "User preferences for notifications")
+
+On the personal settings main page, the user has to accept SMS billing - or be granted free SMSs by the super user (option only visible to super users).
+
+![User preferences for SMS billing](./illustrations/messaging_sms_billing_settings.png "User preferences for SMS billing")
+
+## Code
+The code for the messaging system is found in the `no.nibio.vips.logic.messaging` package and subpackages.
+
+### Usage example
+
 
 ## Configuration
 These system properties must be set in order for the messaging system to function:
 
-```xml
-<property name="no.nibio.vips.logic.messaging.dist.SIMULATE_MAIL_SENDING" value="true"/>
-<property name="no.nibio.vips.logic.messaging.dist.MAIL_SENDER_ADDRESS" value="noreply@foobar.com"/>
-<property name="no.nibio.vips.logic.messaging.dist.SMS_SERVER" value="smsgateway.foobar.com"/>
-<property name="no.nibio.vips.logic.messaging.dist.SMS_USERNAME" value="foobar"/>
-<property name="no.nibio.vips.logic.messaging.dist.SMS_PASSWORD" value="foobar"/>
-<property name="no.nibio.vips.logic.messaging.dist.SMS_SENDER_SRC" value="foobar"/>
-<property name="no.nibio.vips.logic.messaging.dist.TRAFFIC_LOG_DIRECTORY" value="/var/log/trafficLog/"/>
+```bash
+# Set to true if you want to dry run the email system
+no.nibio.vips.logic.messaging.dist.SIMULATE_MAIL_SENDING=true
+# This will be the email sender address displayed to the receiver
+no.nibio.vips.logic.messaging.dist.MAIL_SENDER_ADDRESS=noreply@foobar.com
+# The URL to the SMS provider's API
+no.nibio.vips.logic.messaging.dist.SMS_SERVER=smsgateway.foobar.com
+# Username for the SMS gateway API
+no.nibio.vips.logic.messaging.dist.SMS_USERNAME=foobar
+# Password for the SMS gateway API
+no.nibio.vips.logic.messaging.dist.SMS_PASSWORD=foobar
+# Where the messaging system stores its logs
+no.nibio.vips.logic.messaging.dist.TRAFFIC_LOG_DIRECTORY=/var/log/trafficLog/
+```
+
+How to configure these system properties may vary between application servers. The example below is from WildFly's `[...]/standalone/configuration/standalone.xml` configuration file. 
 
+```xml
+<system-properties>
+    <property name="no.nibio.vips.logic.messaging.dist.SIMULATE_MAIL_SENDING" value="true"/>
+    <property name="no.nibio.vips.logic.messaging.dist.MAIL_SENDER_ADDRESS" value="noreply@foobar.com"/>
+    <property name="no.nibio.vips.logic.messaging.dist.SMS_SERVER" value="smsgateway.foobar.com"/>
+    <property name="no.nibio.vips.logic.messaging.dist.SMS_USERNAME" value="foobar"/>
+    <property name="no.nibio.vips.logic.messaging.dist.SMS_PASSWORD" value="foobar"/>
+    <property name="no.nibio.vips.logic.messaging.dist.SMS_SENDER_SRC" value="foobar"/>
+    <property name="no.nibio.vips.logic.messaging.dist.TRAFFIC_LOG_DIRECTORY" value="/var/log/trafficLog/"/>
+</system-properties>
 ```
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