Better wording about composer dependency

INSTALL

@@ -39,7 +39,7 @@ REQUIREMENTS
   or SQLite support in PHP
 * One of the above databases with permission to create tables
 * An SMTP server (recommended) or PHP configured for mail delivery
-* Composer installed either locally or globally
+* Composer installed either locally or globally (https://getcomposer.org)
 
 
 INSTALLATION

UPGRADING

@@ -40,6 +40,7 @@ it on a unix system, you need to do the following operations by hand:
    directory into the target folder:
    ./skins/
    ./plugins/
+   ./vendor/
 4. Run `./bin/update.sh` from the commandline OR
    open http://url-to-roundcube/installer/ in a browser and choose "3 Test config".
    To enable the latter one, you have to temporary set 'enable_installer'
@@ -47,7 +48,8 @@ it on a unix system, you need to do the following operations by hand:
    WARNING: See SQLite database upgrade below.
 5. Let the update script/installer check your configuration and
    update your config files and database schema as suggested by the updater.
-5. Update dependencies by running `php composer.phar update --no-dev`
+5. If you previously installed plugins through composer, update dependencies
+   by running `php composer.phar update --no-dev`
 7. Make sure 'enable_installer' is set to false again.
 8. See Post-Upgrade Activities section.