Created Data format (markdown)

Data-format.md

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+Boostnote saves documents in a folder such as `~/Boostnote` as [cson](https://github.com/bevry/cson) files.
+
+# data structure
+
+
+```sh
+[ Boostnote 10:01 ]% tree
+.
+├── boostnote.json
+├── images
+│   ├── ay1wuf9yhw0wl8fr.png
+│   ├── qppsh9gs44nghkt9.png
+│   └── 44yt208ehhcnxw29.png
+└── notes
+    ├── 01f720cbd38f6079e7cd.cson
+    ├── 0391ce9b4b6bf0ae8cb7.cson
+    ├── 2b7d62d24dea826d8b86.cson
+    ├── 304a604e620fe29efc0e.cson
+    ├── b9c22f55de5747604fd8.cson
+    ├── d9416376efecccfbfd36.cson
+    └── f7780965124b6d8b8e9e.cson
+
+2 directories, 11 files
+```
+
+## boostnote.json
+The contents of `boostnote.json` is
+
+```json
+{
+  "folders": [
+    {
+      "key": "49e4ebf9f16ddfc96534",
+      "color": "#E10051",
+      "name": "Tutorial"
+    },
+    {
+      "key": "1de44c2b39b9288e7968",
+      "color": "#2BA5F7",
+      "name": "aaaaa"
+    }
+  ],
+  "version": "1.0"
+}
+```
+This is an image of the folder(storage).
+
+![image](https://user-images.githubusercontent.com/11307908/31155778-98f871ac-a8eb-11e7-8f4b-6f922ef52f05.png)
+
+### Naming rule
+Random name for folder is created by 
+
+# Why cson?
+CSON supports multiline string. So we can use diff much easier.
+
+.json
+```
+{
+  content: "a\nlot\nof\lines"
+}
+```
+
+.cson
+```
+  content: """
+    a
+    lot
+    of
+    lines
+  """
+```
+
+ref: https://github.com/BoostIO/Boostnote/issues/89