Added pluggable Avatar providers

doc/plugin-avatar-provider.markdown

@@ -0,0 +1,32 @@
+Adding a new Avatar Provider
+=============================
+
+Registration
+------------
+
+```php
+$this->avatarManager->register(new CustomAvatarProvider());
+```
+
+Interface
+---------
+
+The provider must implements the interface `Kanboard\Core\User\Avatar\AvatarProviderInterface`:
+
+
+| Method                        | Description                                                   |
+|-------------------------------|---------------------------------------------------------------|
+| `render(array $user, $size)`  | Render HTML                                                   |
+| `isActive(array $user)`       | Returns a boolean if the provider is able to render something |
+
+
+The `$user` argument is a dictionary that contains these keys:
+
+```php
+[
+    'id' => 123,
+    'username' => 'admin',
+    'name' => 'Administrator',
+    'email' => 'me@localhost',
+]
+```