Update 'Usage'

Usage.md

@@ -1 +1,17 @@
-Welcome to the Wiki.
+## Basic usage
+
+Using Fresh is as simple as falling off a log. Just run it while in a directory that contains a `docker-compose.yml` file, and Fresh will go and check for updates to any registries you have authenticated against. This means it currently can't be used to check images from dockerhub, but it works great for private registries.
+
+### Slack/Mattermost Webhooks
+
+Fresh can invoke a Slack-compatible webhook to notify before anything is actually updated. To do this, use the `--slack` flag followed by the webhook URL
+
+### Script hooks
+
+You can specify a script to run after an update by using the `--after` flag.
+
+## Scheduled run
+
+### Using cron
+
+### Using Systemd