Merge branch 'swappiness-req-doc' into 'master'

Document swappiness recomendations in the requirements doc

See merge request !13943

doc/install/requirements.md

@@ -82,7 +82,9 @@ errors during usage.
 
 We recommend having at least 2GB of swap on your server, even if you currently have
 enough available RAM. Having swap will help reduce the chance of errors occurring
-if your available memory changes.
+if your available memory changes. We also recommend [configuring the kernels swappiness setting](https://askubuntu.com/a/103916)
+to a low value like `10` to make the most of your RAM while still having the swap
+available when needed.
 
 Notice: The 25 workers of Sidekiq will show up as separate processes in your process overview (such as top or htop) but they share the same RAM allocation since Sidekiq is a multithreaded application. Please see the section below about Unicorn workers for information about many you need of those.