docs(slack): added slack appender docs

docs/slack.md

@@ -0,0 +1,31 @@
+# Slack Appender
+
+Sends log events to a [slack](https://slack.com) channel. To use this appender you will need to include [slack-node](https://www.npmjs.com/package/slack-node) in your application's dependencies.
+
+## Configuration
+
+* `type` - `slack`
+* `token` - `string` - your Slack API token (see the slack and slack-node docs)
+* `channel_id` - `string` - the channel to send log messages
+* `icon_url` - `string` (optional) - the icon to use for the message
+* `username` - `string` - the username to display with the message
+* `layout` - `object` (optional, defaults to `basicLayout`) - the layout to use for the message (see [layouts](layouts.md)).
+
+## Example
+
+```javascript
+log4js.configure({
+  appenders: {
+    alerts: {
+      type: 'slack',
+      token: 'abc123def',
+      channel_id: 'prod-alerts',
+      username: 'our_application'
+    }
+  },
+  categories: {
+    default: { appenders: ['alerts'], level: 'error' }
+  }
+});
+```
+This configuration will send all error (and above) messages to the `prod-alerts` slack channel, with the username `our_application`.