README: update with info about conf file

README

@@ -108,6 +108,7 @@ recommended that you start a separate shell with root privileges while
 configuring PAM to be able to revert changes if something goes wrong.
 Test your configuration thoroughly before closing the root shell.
 
+[[moduleArguments]]
 === Module Arguments
 
 [horizontal]
@@ -240,6 +241,14 @@ FIDO devices. It is not possible to mix native credentials and SSH
 credentials. Once this option is enabled all credentials will be parsed
 as SSH.
 
+conf=/path/to/pam_u2f.conf::
+Set an alternative location for the <<confFile,configuration file>>.
+The supplied path must be absolute and must correspond to an existing
+regular file.
+
+The options specified on the module command line override the values
+from the <<confFile,configuration file>>.
+
 IMPORTANT: On dynamic networks (e.g. where hostnames are set by DHCP),
 users should not rely on the default origin and appid
 ("pam://$HOSTNAME") but set those parameters explicitly to the same
@@ -404,6 +413,29 @@ defined in the authorization mapping file. If during an authentication attempt
 a connected device is removed or a new device is plugged in, the authentication
 restarts from the top of the list.
 
+[[confFile]]
+== Configuration file
+
+A configuration file can be used to set the default
+<<moduleArguments,module arguments>>.
+
+- Arguments appear one per line.
+
+- Any `conf_file=` argument in the configuration file is ignored.
+
+- Lines starting with `#` are interpreted as comments.
+
+- Leading white space is ignored. Everything from the first
+  non-white-space character until the end of line is interpreted verbatim.
+  For this reason, trailing comments are disallowed.
+
+- The maximum size for the configuration file is 4 KiB.
+
+The default path for the configuration file is
+`/etc/security/pam_u2f.conf`. Note that it may have been set to another
+value by the distribution. The default file is allowed to not exist. An
+alternative path may be set in the module command line options.
+
 == SELinux Note
 
 Due to an issue with Fedora Linux, and possibly with other