Add contribution guide

contribution.md

@@ -0,0 +1,28 @@
+# Contribution
+
+## Brief introduction to the project
+
+```
+.
+└── Contents
+    └── Sketch
+        ├── checkupdate.cocoascript
+        ├── export.cocoascript
+        ├── index.html
+        ├── manifest.json
+        ├── util.cocoascript
+        └── zip.cocoascript
+```
+
+The project can be devided into two major parts.
+
+First of which is the CocoaScript part. We grub the information we need from Sketch using the [offical API](http://bohemiancoding.com/sketch/support/developer/01-introduction/01.html) and store it into a json file, which can be found in every zip file you export.
+
+The second part is the all-in-one `index.html`. It uses the json data and displays the images and layers. It really just normal front-end stuff, which means you would deal with HTML, CSS and JavaScript.
+
+## Bug targeting guide
+
+Since the `index.html` is just responsible for displaying, if you find any bugs related to information, such as counting extra 1 pixel here and there, it mostly is the cocoascript's bad. And of course when it comes to the UI problem, you should go and see the HTML file.
+
+If you have any questions, feel free to open an issue.
+