Added side note about enabling gzip on Koa (vercel#2867)

I'll share this small snippet here, it might be useful.

Author: mmazzarolo

examples/custom-server-koa/README.md

@@ -31,3 +31,20 @@ Most of the times the default Next server will be enough but sometimes you want
 Because the Next.js server is just a node.js module you can combine it with any other part of the node.js ecosystem. in this case we are using [Koa](http://koajs.com/) to build a custom router on top of Next.
 
 The example shows a server that serves the component living in `pages/a.js` when the route `/b` is requested and `pages/b.js` when the route `/a` is accessed. This is obviously a non-standard routing strategy. You can see how this custom routing is being made inside `server.js`.
+
+
+## Side note: Enabling gzip compression
+
+The most common Koa middleware for handling the gzip compression is [compress](https://github.com/koajs/compress), but unfortunately it is currently not compatible with Next.  
+`koa-compress` handles the compression of the response body by checking `res.body`, which will be empty in the case of the routes handled by Next (because Next sends and ends the response by itself). 
+
+If you need to enable the gzip compression, the most simple way to do so is by wrapping the express-middleware [compression](https://github.com/expressjs/compression) with [koa-connect](https://github.com/vkurchatkin/koa-connect):  
+
+```javascript
+const compression = require('compression');
+const koaConnect = require('koa-connect');
+
+
+server.use(koaConnect(compression()));
+
+```