Document the site-logo configuration option

The configuration option for site logo wasn't well documented, so I
added it to the README file, config.php.sample, and common.php.

README

@@ -884,6 +884,8 @@ notice: A plain string that will appear on every page. A good place
         be escaped.
 dupelimit: Time in which it's not OK for the same person to post the
            same notice; default = 60 seconds.
+logo: URL of an image file to use as the logo for the site. Overrides
+      the logo in the theme, if any.
 
 db
 --

config.php.sample

@@ -37,6 +37,9 @@ $config['site']['path'] = 'laconica';
 # Enables extra log information, for example full details of PEAR DB errors
 #$config['site']['logdebug'] = true;
 
+#To set your own logo, overriding the one in the theme
+#$config['site']['logo'] = '/mylogo.png';
+
 # This is a PEAR DB DSN, see http://pear.php.net/manual/en/package.database.db.intro-dsn.php
 # Set it to match your actual database
 

lib/common.php

@@ -73,6 +73,7 @@ $config =
               'theme' => 'default',
               'path' => $_path,
               'logfile' => null,
+              'logo' => null,
               'logdebug' => false,
               'fancy' => false,
               'locale_path' => INSTALLDIR.'/locale',