Add usage to readme

Author: tulir

Diff for: README.md

@@ -1,2 +1,30 @@
 # rss
 A [maubot](https://github.com/maubot/maubot) that posts RSS feed updates to Matrix.
+
+## Usage
+Basic commands:
+
+* `!rss subscribe <url>` - Subscribe the current room to a feed.
+* `!rss unsubscribe <feed ID>` - Unsubscribe the current room from a feed.
+* `!rss subscriptions` - List subscriptions (and feed IDs) in the current room.
+* `!rss notice <feed ID> [true/false]` - Set whether the bot should send new
+  posts as `m.notice` (if false, they're sent as `m.text`).
+* `!rss template <feed ID> [new template]` - Change the post template for a
+  feed in the current room. If the new template is omitted, the bot replies
+  with the current template.
+
+### Templates
+The default template is `New post in $feed_title: [$title]($link)`.
+
+Templates are interpreted as markdown with some simple variable substitution.
+The following variables are available:
+
+* `$feed_url` - The URL that was used to subscribe to the feed.
+* `$feed_link` - The home page of the feed.
+* `$feed_title` - The title of the feed.
+* `$feed_subtitle` - The subtitle of the feed.
+* `$id` - The unique ID of the entry.
+* `$date` - The date of the entry.
+* `$title` - The title of the entry.
+* `$summary` - The summary/description of the entry.
+* `$link` - The link of the entry.