Another README update

README.md

@@ -1,3 +1,10 @@
 # Pixivbot
 This is a Slack bot based on [slack-ruby-bot](https://github.com/dblock/slack-ruby-bot), [slack-mathbot](https://github.com/dblock/slack-mathbot), and inspired by another [Slack Pixiv Bot](https://github.com/castaneai/slack-pixiv-bot).
 
+## Purpose
+
+When a [Pixiv](http://www.pixiv.net/) link is posted in the channel the Pixiv bot is in, it will "unfurl" the image via message attachements. Pixiv does not allow the images on the site to reside in the metadata, so standard Slack unfurling is unable to preview the image.
+
+## License
+
+Do what you want. It's a Slack Bot.