Doc: Add docker and development how to for plugins

docs/src/plugin-system.md

@@ -1,15 +1,39 @@
 # New plugin system for Pinry
 
-New plugin system is under development.
+New plugin system is under development and a naive version has been released.
 
-Now you could access it via project example plugin file `pinry_plugins/batteries/plugin_example.py`.
+A `PinryPlugin` is a python class or object which is callable.
+The plugin loader will call the plugin argument only once and use the plugin
+instance after specified events triggered just like the way django-middleware works.
 
-You could create a simple plugin which has a class which owns methods named:
+You could create a plugin as python-package with content:
 
-+ process_image_pre_creation
-+ process_thumbnail_pre_creation
- 
-And, add the plugin class to local_settings.py as:
+```
+from core.models import Image
+from django_images.models import Thumbnail
+
+
+class Plugin:
+    
+    def __init__(self):
+        # do something you want, just be called only
+        pass
+
+    def process_image_pre_creation(self, django_settings, image_instance: Image):
+        pass
+
+    def process_thumbnail_pre_creation(self, django_settings, thumbnail_instance: Thumbnail):
+        pass
+
+```
+
+
+You could make some changes on Image object and Thumbnail object 
+before they actually be saved (for example, add water-mark to them).
+
+You could access example plugin via `pinry_plugins/batteries/plugin_example.py`.
+
+After all, enable the plugin in local_settings.py:
 
 ```
 ENABLED_PLUGINS = [
@@ -18,3 +42,24 @@ ENABLED_PLUGINS = [
 ```
 
 Now the plugin will work like a charm!
+
+# List of Available Plugins
+
+left blank to fill, coming soon...
+
+
+# Install Plugin in Docker
+If you have some plugin which named `hello.py` and a have a `Plugin` class inside.
+
+You could just copy them to directory `pinry_plugins/batteries`.
+
+Now add config to local_settings.py
+
+```
+ENABLED_PLUGINS = [
+    'pinry_plugins.batteries.hello.Plugin',
+]
+```
+
+Then, rebuild your docker image, the plugin will work 
+if no further python dependencies required.