Updated Design: Persistable Context (markdown)

Design:-Persistable-Context.md

@@ -152,6 +152,77 @@ It executes plugin specific function passed in the argument `command`.
 
 **Returns** a value that plugin specific function returns. If the plugin return nothing like a `set`, `run` will return `undefined`.
 
+## LocalFileSystem plugin  
+　　The LocalFileSystem plugin is one of the bundle plugins for Persistable Context.  
+　　This plugin stores context to Local File System.
+
+<details>
+
+### Directory Structure
+```
+$HOME/.node-red/contexts
+├── global
+│     └── global_context.json
+├── <id of Flow 1>
+│     ├── flow_context.json
+│     ├── <id of Node a>.json
+│     └── <id of Node b>.json
+└── <id of Flow 2>
+       ├── flow_context.json
+       ├── <id of Node x>.json
+       └── <id of Node y>.json
+```
+- The plugin creates `settings.userDir`/contexts directory if `config.dir` is not passed.
+- `global context` is stored in global directory as global_context.json.
+- `flow context` is stored in \<id of the flow> directory as flow_context.json.  
+- `local context` is stored in \<id of the flow it is in> directory as \<id of the node>.json.    
+
+#### example 
+
+```
++--------+
++ Flow A +
++--------+----------------------------+
++                                     +
++　+----------+        +----------+   +
++  +timestamp +--------+ function +   +
++　+----------+        +----------+   +
++                                     +
++-------------------------------------+
+```
+- The id of the Flow A is `8588e4b8.784b38`
+- The id of the function node is `80d8039e.2b82`
+- The function node stores data to local, flow and global context in local file system.
+- The following directories and files are created after the Flow A runs.
+```
+.node-red/contexts
+├── global
+│     └── global_context.json
+└── 8588e4b8.784b38
+       ├── flow_context.json
+       └── 80d8039e.2b82.json
+
+```
+
+### File Format
+- The plugin stores data as formatted JSON.
+
+```
+{
+    "key": "value", 
+    "num": 123456, 
+    "array": [
+        "1", 
+        "2", 
+        "3"
+    ], 
+    "object": {
+        "foo": "bar"
+    }
+}
+```
+</details>
+
 ## Considerations
 
 #### Serializing/Deserializsing JavaScript Objects