Create CONTRIBUTING.md

Author: ilopX

CONTRIBUTING.md

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+## Contributor's Guide
+We appreciate any help, whether it's a simple fix of a typo or a whole new example. 
+Just [make a fork](https://help.github.com/articles/fork-a-repo/), 
+do your change and submit a [pull request](https://help.github.com/articles/creating-a-pull-request-from-a-fork/).
+
+### Step-by-step instruction
+1. Just make a fork.
+2. Clone the forked repository to your local machine.
+3. Create a new branch and name it, for example: fix-issue-32.
+4. Make changes.
+5. Create commits and push them to your forked Github repository.
+6. Submit a pull request to the master branch.
+7. Wait for review.
+
+### Style guide
+Here's a style guide which might help you to keep your changes consistent with our code:
+
+1. All code should meet the [Effective Dart: Style](https://dart.dev/guides/language/effective-dart/style).
+ 
+2. Use [Dart Format](https://dart.dev/tools/dart-format) or auto format shortcut `Ctrl + Alt + L` in your ide. 
+
+3. Try to hard wrap the code at 80th's character. It helps to list the code on the website without scrollbars.
+
+4. File names should match following convention: `some_class_name.dart`
+
+5. Comments may or may not have language tags in them, such as this:
+    ```dart
+        // EN: All products families have the same varieties (MacOS/Windows).
+        // This is a MacOS variant of a button.
+        
+        // RU: Все семейства продуктов имеют одни и те же вариации (MacOS/Windows).
+        // Это вариант кнопки под MacOS.
+    ```
+    Don't be scared and ignore the non-English part of such comments. If you want to change 
+    something in a comment like this, then do it. Even if you do it wrong, we'll tell you how 
+    to fix it during the Pull Request.
+
+
+### Build Flutter examples
+```batch
+cd root directory
+flutter build web -t bin\main.dart
+```
+
+### Deploy flutter demos
+1. Fork this repo: `https://github.com/RefactoringGuru/design-patterns-dart`
+2. Apply your changes.
+3. Run the script `dart bin\deploy_flutter_demos.dart`.
+This script will build a web platform flutter app and push the changes to your **web-demos** branch on github.
+4. You can now make a pull request on the **web-demos** branch.
+5. Once approved for the merge, the web app will be available at https://refactoringguru.github.io/design-patterns-dart .