The procedure to create a OS X El Capitan recovery usb key is the same as it was for Yosemite:
To create your OS X El Capitan recovery key you need to download the installer from the App Store. After the download has finished quit the installer and plug in your USB key (at least 8 gb). Now you can create your recovery key with the following command.
be aware to use the correct volume name
sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/*VOLNAME* --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app --nointeraction