Update README.md

Author: rauchg

README.md

@@ -36,7 +36,7 @@ Read more about the [motivations and history](http://rauchg.com/slackin) behind
 
 For other CloudFoundry providers, [follow these instructions.](https://github.com/pivotal-cf/slackin/wiki/Cloud-Foundry)
 
-#### Custom
+#### NPM
 
 Install it and launch it on your server:
 
@@ -45,6 +45,18 @@ $ npm install -g slackin
 $ slackin "your-team-id" "your-slack-token"
 ```
 
+#### Node.js (programmatic use)
+
+```
+const slackin = require('slackin').default;
+// slackin returns a `http.Server`
+// `server.app` is the underlying express app
+const server = slackin({
+  /* options */
+});
+server.listen(3000);
+```
+
 Your team id is what you use to access your login page on Slack (eg: https://**{this}**.slack.com).
 
 You can find or generate your API test token at [api.slack.com/web](https://api.slack.com/web) – note that the user you use to generate the token must be an admin. You need to create a dedicated `@slackin-inviter` user (or similar), mark that user an admin, and use a test token from that dedicated admin user.  Note that test tokens have actual permissions so you do not need to create an OAuth 2 app. Also check out the Slack docs on [generating a test token](https://get.slack.help/hc/en-us/articles/215770388-Creating-and-regenerating-API-tokens).