Helper refactoring

doc/plugin-helpers.markdown

@@ -0,0 +1,40 @@
+Registering new helpers
+=======================
+
+Helper skeleton:
+
+```php
+<?php
+
+namespace Kanboard\Plugin\MyPlugin\Helper\MyHelper;
+
+use Kanboard\Core\Base;
+
+class MyHelper extends Base
+{
+    public function doSomething()
+    {
+        return 'foobar';
+    }
+}
+```
+
+Register your helper class:
+
+```php
+$this->helper->register('myHelper', '\Kanboard\Plugin\MyPlugin\Helper\MyHelper');
+```
+
+Using your helper from a template:
+
+```php
+<p>
+    <?= $this->myHelper->doSomething() ?>
+</p>
+```
+
+Using your helper from another class:
+
+```php
+$this->helper->myHelper->doSomething();
+```