add contribution guidelines for icon, thumbnail and screenshots (#163)

Signed-off-by: Tiger Wang <tigerwang@outlook.com>
Co-authored-by: 老竭力 <jerrykuku@qq.com>

CONTRIBUTING.md

@@ -4,7 +4,32 @@ This document describes how to contribute an app to CasaOS AppStore.
 
 ## Guidelines
 
-A CasaOS App is a Docker Compose app, or a *compose app*.
+### Project Structure
+
+```bash
+CasaOS-AppStore
+├─ category-list.json   # Configuration file for category list
+├─ recommend-list.json  # Configuration file for recommended apps list
+├─ featured-apps.json   # TBD
+├─ help                 # Help script for old version app store
+├─ Apps                 # Apps Store files
+├─ build                # Installation script for Apps Store
+└─ psd-source           # Icon thumbnail screenshot PSD Templates
+```
+
+### A complete app profile should include the following contents
+
+```bash
+App-Name
+├─ docker-compose.yml   # A valid Docker Compose file
+├─ icon.png             # App icon
+├─ thumbnail.png        # Required when the app is used as a recommended app
+├─ screenshot-1.png     # Three screenshots introducing the app are required in the app details
+├─ screenshot-2.png
+└─ screenshot-3.png
+```
+
+#### A CasaOS App is a Docker Compose app, or a *compose app*
 
 Each directory under [Apps](Apps) correspond to a CasaOS App. The directory should contain at least a `docker-compose.yml` file:
 
@@ -95,3 +120,11 @@ Each directory under [Apps](Apps) correspond to a CasaOS App. The directory shou
             index: /                        # the index page for web UI, e.g. index.html
             port_map: "8384"                # the port for web UI
         ```
+
+#### Specifications of Icon, Thumbnail and Screenshots
+
+- Icon image should be a transparent background PNG image with a size of 192x192 pixels.
+- Thumbnail image should be 784x442 pixels, with a rounded corner mask. It is recommended to be saved as a PNG image with a transparent background.
+- Screenshot image should be 1280x720 pixels and can be saved in either PNG or JPG format. Please try to keep the file size as small as possible.
+
+Please find the prepared [PSD template files](psd-source), to quickly create the above images if you need.