readme: add sub-modules usage instructions.

Author: nunofmn

README.md

@@ -25,6 +25,7 @@ ipfs-api
 - [Install](#install)
   - [Running the daemon with the right port](#running-the-daemon-with-the-right-port)
   - [Importing the module and usage](#importing-the-module-and-usage)
+  - [Importing a sub-module and usage](#importing-a-sub-module-and-usage)
   - [In a web browser through Browserify](#in-a-web-browser-through-browserify)
   - [In a web browser from CDN](#in-a-web-browser-from-cdn)
   - [CORS](#cors)
@@ -74,6 +75,14 @@ var ipfs = ipfsAPI('/ip4/127.0.0.1/tcp/5001')
 // or using options
 var ipfs = ipfsAPI({host: 'localhost', port: '5001', protocol: 'http'})
 ```
+### Importing a sub-module and usage
+```javascript
+const bitswap = require('ipfs-api/bitswap')('/ip4/127.0.0.1/tcp/5001')
+
+bitswap.unwant(key, (err) => {
+  // ...
+}
+```
 
 ### In a web browser through Browserify