added webpack-encore for managing frontend assets #113 (#122)

* updated installation docu #113
* rewritten login page #113
* added icheck plugin #113
* changed to YesNoType in edit forms #113
* updated CONTRIBUTING.md #113
* added start of developer docu #113
* added compile scripts and compiled assets #113

var/docs/developers.md

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+# Developers
+
+This page is dedicated to all developers who want to contribute to Kimai. You are the best!
+
+# Setting up your environment
+
+All you need is:
+- PHP >= 7.1 
+- PHP extension: `PDO-SQLite` enabled
+
+Optional requirement:
+- a MySQL/MariaDB instance
+- PHP extension: `PDO-MySQL` enabled
+
+You could even test PostgreSQL and tell us how it works!
+
+## Frontend dependencies 
+
+If you want to make changes to CSS / Javascripts, you need:
+
+- [NodeJS](https://www.npmjs.com/)
+- [Yarn Package Manager](https://yarnpkg.com/en/)
+- [Webpack](https://webpack.js.org/)
+- [Webpack Encore](https://github.com/symfony/webpack-encore)
+
+Please [install Yarn for your OS](https://yarnpkg.com/lang/en/docs/install/) and then:
+
+```bash
+yarn install
+```
+
+To rebuild all assets you have to execute:
+```bash
+npm run dev
+```
+or
+```bash
+npm run prod
+```
+
+You can find more information at:
+
+- https://symfony.com/doc/current/frontend/encore/installation.html
+- https://symfony.com/doc/current/frontend.html
+
+### Rebuilding assets for use in subdirectory
+
+If you want to run Kimai in a subdirectory, you have to rebuild the frontend assets with a different webpack configuration.
+Edit the file [webpack.config.js](https://github.com/kevinpapst/kimai2/blob/master/webpack.config.js) and change `.setPublicPath('/build/')` to your needs.
+
+After that re-compile the assets with:
+```bash
+npm run prod
+```
+
+## Extending the navigation bar
+
+If you want to add your own entries in the navigation bar, you can subscribe to these events:
+
+- `App\EventConfigureMainMenuEvent::CONFIGURE`
+- `App\ConfigureAdminMenuEvent::CONFIGURE`
+
+And that's how to use it:
+
+```php
+use App\Event\ConfigureMainMenuEvent;
+use App\Event\ConfigureAdminMenuEvent;
+use Avanzu\AdminThemeBundle\Model\MenuItemModel;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+class MySubscriber implements EventSubscriberInterface
+{
+    public static function getSubscribedEvents(): array
+    {
+        return [
+            ConfigureMainMenuEvent::CONFIGURE => ['onMainMenuConfigure', 100],
+            ConfigureAdminMenuEvent::CONFIGURE => ['onAdminMenuConfigure', 100],
+        ];
+    }
+    
+    public function onMainMenuConfigure(ConfigureMainMenuEvent $event)
+    {
+        $event->getMenu()->addItem(
+            new MenuItemModel('timesheet', 'menu.timesheet', 'timesheet', [], 'fa fa-clock-o')
+        );
+    }
+
+    public function onAdminMenuConfigure(ConfigureAdminMenuEvent $event)
+    {
+        $event->getAdminMenu()->addChild(
+            new MenuItemModel('timesheet_admin', 'menu.admin_timesheet', 'admin_timesheet', [], 'fa fa-clock-o')
+        );
+    }    
+}
+```
+For more details check the [official menu subscriber](https://github.com/kevinpapst/kimai2/blob/master/src/EventSubscriber/MenuSubscriber.php).
+
+## Adding tabs to the "control sidebar"
+
+We use twig globals to render the control sidebar tabs, so adding one is as easy as adding a new config entry:
+
+```yaml
+twig:
+    globals:
+        kimai_context:
+            control_sidebar:
+                # these are the official tabs
+                settings:
+                    icon: gears
+                    controller: 'App\Controller\SidebarController::settingsAction'
+                home:
+                    icon: question-circle-o
+                    template: sidebar/home.html.twig
+
+```
+You have to define the icon to be used and then one of controller action or template. 
+Both follow the default naming syntax and you can easily link your bundle here instead of the official.
+You should NOT add them in `config/packages/kimai.yaml` but only in your own bundle config.