Instructions in README for setting up Discord integration (#138)

README.md

@@ -217,6 +217,20 @@ backups set up.
 
 ## Integrations
 
+### Discord
+
+To enable Discord integration, you will need to:
+
+* register a new application on https://discordapp.com/developers/applications/me
+* add a redirect URI to your Discord application. The URI format is
+  `SITE_ROOT/integrations/add_discord/`. For example, if you are running a
+  development server on `localhost:8000` then the redirect URI would be
+  `http://localhost:8000/integrations/add_discord/`
+* Look up your Discord app's Client ID and Client Secret. Add them
+  to your `hc/local_settings.py` file as `DISCORD_CLIENT_ID` and
+  `DISCORD_CLIENT_SECRET` fields.
+
+
 ### Pushover
 
 To enable Pushover integration, you will need to: