Add note about -c option

RELEASENOTES.md

@@ -12,7 +12,8 @@ When `make install` is executed, a sample configuration is installed or updated
 If you enable metadata, please note that the option has changed somewhat. The option `-M` has a new long name equivalent: `--metadata-pipename` and the argument you provide must now be the full name of the metadata pipe, e.g. `-M /tmp/shairport-sync-metadata`.
 
 * Enhancements
- * Shairport Sync now reads the configuration file `/etc/shairport-sync.conf`. This has settings for most command-line options and it's where any new settings will go. A default configuration file will be installed if one doesn't exist, and a sample file configuration file is always installed or updated. Details of settings are provided in the sample file. Shairport Sync relies on the `libconfig` library to read configuration files. For the present, you can disable the new feature (and save the space taken up by `libconfig`) by using the configure option `--without-config-file-support`.
+ * Shairport Sync now reads settings from the configuration file `/etc/shairport-sync.conf`. This has settings for most command-line options and it's where any new settings will go. A default configuration file will be installed if one doesn't exist, and a sample file configuration file is always installed or updated. Details of settings are provided in the sample file. Shairport Sync relies on the `libconfig` library to read configuration files. For the present, you can disable the new feature (and save the space taken up by `libconfig`) by using the configure option `--without-config-file-support`.
+ * New command-line option `-c <file>` or `--configfile=<file>` allows you to specify a configuration file other than `/etc/shairport-sync.conf`.
  * Session Timeout and Allow Session Interruption can now be set independently. This is really some "housekeeping" as referred to above -- it's a kind of a bug fix, where the bug in question is an inappropriate connection of the setting of two parameters. To explain: (1) By default, when a source such as iTunes starts playing to the Shairport Sync device, any other source attempting to start a play session receives a "busy" signal. If a source disappears without warning, Shairport Sync will wait for 120 seconds before dropping the session and allowing another source to start a play session. (2) The command-line option `-t` or `--timeout` allows you to set the wait time before dropping the session. If you set this parameter to `0`, Shairport Sync will not send a "busy" signal, thus allowing another source to interrupt an existing one. (3) The problem is that if you set the parameter to `0`, a session will never be dropped if the source disappears without warning.
  The (obvious) fix for this is to separate the setting of the two parameters, and this is now done in the configuration file `/etc/shairport-sync.conf` -- please see the settings `allow_session_interruption` and `session_timeout`. The behaviour of the `-t` and `--timeout` command-line options is unchanged but deprecated.
  * New Option -- "Ignore Volume Control". If you set this to "yes", the output from Shairport Sync is always set at 100%. This is useful when you want to set the volume locally. Available via the settings file only.