docs: add contributing.md

CONTRIBUTING.md

@@ -0,0 +1,27 @@
+# Contributing
+
+## Repository structure
+
+```sh
+lib
+├── main.dart # App entry
+├── graphql
+│   ├── # GraphQL queries
+├── models
+│   ├── # Provider models and other JSON models
+├── router.dart # Routers here
+├── scaffolds
+│   ├── # Several scaffolds for reducing boilerplate code
+├── screens
+│   ├── gh_xxx.dart # GitHub screens
+│   ├── bb_xxx.dart # Bitbucket screens
+│   ├── gl_xxx.dart # GitLab screens
+│   ├── gt_xxx.dart # Gitea screens
+│   └── # File with no prefix: common screens
+├── utils
+│   ├── # Utilities
+└── widgets
+    └── # Reusable widgets
+```
+
+Feel free to open an issue to discuss it if you get stuck.

README.md

@@ -24,6 +24,10 @@ Android users can also download APK file from [release page](https://github.com/
   <img src="assets/screenshot-light.png" alt="Light screenshot" width="400" />
 </p>
 
+## Contributing
+
+See [CONTRIBUTING.md](./CONTRIBUTING.md).
+
 ## Contributors
 
 ![](https://sourcerer.io/fame/pd4d10/pd4d10/git-touch/images/0)