Add vendors/ directory for third-party modules

- The odd-looking .gitignore is so that the README file is included in the
repository, but if the user installs some third part modules, they won't
accidentally be added to the skeleton git repository. We do the same thing in
the modules/ directory.
- Added readme explaining that vendors/ is for third-party modules

configs/application.config.php

@@ -2,6 +2,7 @@
 return array(
     'module_paths' => array(
         realpath(__DIR__ . '/../modules'),
+        realpath(__DIR__ . '/../vendors'),
     ),
     'modules' => array(
         'Application',

vendors/.gitignore

@@ -0,0 +1,3 @@
+/*
+!README.md
+!.gitignore

vendors/README.md

@@ -0,0 +1 @@
+This `vendors/` directory is where third-party modules should be installed.