This module implementing XEP-0215 is included with Prosody 0.12 but made available here for older versions.
Prosody 0.11.x is unmaintained. Use mod_external_services for newer prosody versions. Do not use this community module.
Documentation is available on the Prosody site.
With the plugin installer in Prosody 0.12 you can use:
sudo prosodyctl install --server=https://modules.prosody.im/rocks/ mod_external_servicesFor earlier versions see the documentation for installing 3rd party modules