Mention command for generating encryption keys (elastic#508)

tab-widgets/prereq.asciidoc

@@ -29,9 +29,11 @@ service] must be enabled.
 (`xpack.security.authc.api_key.enabled: true`)
 
 * In the {kib} configuration, the saved objects encryption key
-(`xpack.encryptedSavedObjects.encryptionKey`) must be set to any alphanumeric
-value of at least 32 characters. {fleet} requires this setting in order to save
-API keys and encrypt them in {kib}.
+must be set. {fleet} requires this setting in order to save API keys and encrypt
+them in {kib}. You can either set `xpack.encryptedSavedObjects.encryptionKey` to
+an alphanumeric value of at least 32 characters, or run the
+{kibana-ref}/kibana-encryption-keys.html[`kibana-encryption-keys` command] to
+generate the key. 
 
 **Example security settings**