all: implement notification using Mattermost incoming webhook

In the server configuration, one can defined the following section
to send the notification using Mattermost,

[notif]
kind = mattermost
webhook_url = # The incoming webhook URL.
channel = # The channel where the notification will be placed.
down_template = # Message template when service is down.
up_template = # Message template when service is up.

1 files changed, 49 insertions, 0 deletions

diff --git a/README.md b/README.md
new file mode 100644
index 0000000..27b26fa
--- /dev/null
+++ b/README.md
@@ -0,0 +1,49 @@
+# lilin
+
+## Notification
+
+Lilin support sending notification to,
+
+- Mattermost using incoming webhook.
+
+See the next section on how to use the notification.
+
+### Mattermost incoming webhook
+
+In the server configuration, add the section "notif" with the following
+format,
+
+```
+[notif]
+kind = mattermost
+webhook_url = # The incoming webhook URL.
+channel = # The channel where the notification will be placed.
+down_template = # Message template when service is down.
+up_template = # Message template when service is up.
+```
+
+The `down_template` and `up_template` can contains the following variables,
+
+- .ID: the service ID
+- .At: the time when service up or down
+- .Error: the error message that cause the service marked as down
+
+For example, given the following scan report value,
+
+```
+.ID: http-server
+.At: 2025-09-26 06:38:11 +0000 UTC
+.Error: 503 Service Unavailable
+```
+
+The following `down_template`
+
+```
+{{.At}}: Service {{.ID}} is down: {{.Error}}
+```
+
+will be rendered as
+
+```
+2025-09-26 06:38:11 +0000 UTC: Service http-server is down: 503 Service Unavailable
+```