Feature: Add doc for plugin-system

docs/src/plugin-system.md

@@ -0,0 +1,20 @@
+# Pinry, the open-source tiling image board.
+
+New plugin system is under development.
+
+Now you could access it via project example plugin file `pinry_plugins/batteries/plugin_example.py`.
+
+You could create a simple plugin which has a class which owns methods named:
+
++ process_image_pre_creation
++ process_thumbnail_pre_creation
+ 
+And, add the plugin class to local_settings.py as:
+
+```
+ENABLED_PLUGINS = [
+    'pinry_plugins.batteries.plugin_example.Plugin',
+]
+```
+
+Now the plugin will work like a charm!

mkdocs.yml

@@ -19,6 +19,7 @@ nav:
   - Theories: 'theories.md'
   - Install with Docker: 'install-with-docker.md'
   - Development: 'development.md'
+  - Development: 'plugin-system.md'
   - Docs: 'docs.md'
   - Passwords: 'passwords.md'
   - Assets: 'assets.md'