Create CONTRIBUTING.md

Add Internationalization Guide

Author: RockerFlower

CONTRIBUTING.md

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+# Contributing to POAPin
+
+⌐◨-◨- Thank you for your interest in POAPin, and please join us in making it better!
+
+## Internationalization
+
+We wish people all over the world to be able to use POAPin in a language they are familiar with. No one can master all languages, and it will take a long time to get the POAPin app to support multiple languages, but we believe we can get there soon with everyone's help!
+
+Here's an example where you don't need to know how to code and can complete the entire task on the web. Very simple.
+
+1. ### Locate the language file that you are familiar with ###
+
+    Visit this folder: [`lib/common/translations/languages`](https://github.com/glorylab/POAPin/tree/master/lib/common/translations/languages)
+    
+    <img src='https://user-images.githubusercontent.com/5019152/185751834-25c56510-6777-41d3-b82c-2c7d624e8f7e.png' height="360" />
+    
+    Here are the meanings of these language file names:
+    
+    https://github.com/glorylab/POAPin/blob/104f3aa3320b1b1a4ff27669eed973bac57a083f/lib/common/translations/locale_string.dart#L27-L40
+    
+2. ### Open the language you want to contribute to ###
+
+    Here is an example in `🇯🇵Japanese🇯🇵`, let's open [`ja.dart`](https://github.com/glorylab/POAPin/blob/master/lib/common/translations/languages/ja.dart)
+    
+    Click the Edit button to enter the online editor.
+    
+    <img src='https://user-images.githubusercontent.com/5019152/185752536-39702ea4-5112-47e9-8125-428dc4ae240c.png' height="200" />
+    
+    Find the text you can translate, remove the `//` at the beginning of a line and replace the second half of the line with the translation for the corresponding language. Replace `Home` with `ホーム`, for example:
+    
+    <img src='https://user-images.githubusercontent.com/5019152/185752776-69f864f9-fc55-468e-97bd-ee0a75fc945e.png' height="100" />
+    
+    That's it!
+    
+3. ### Commit your file ###
+
+    <img src='https://user-images.githubusercontent.com/5019152/185752922-10b240d2-b90c-4629-94d6-d8b39c7ff96c.png' height="300" />
+
+    Select **Create a New Branch**, as shown in the figure. And **Propose changes**
+
+    <img height="300" src="https://user-images.githubusercontent.com/5019152/185753115-ac9bca3f-073a-4431-9163-55bbba36569f.png">
+    
+    Next, **Draft pull request** or **Create pull request**. If you want to keep editing, don't want to be merged right away, choose Draft.
+    
+4. ### You made it! ###
+
+
+    If the translation is good, your contribution will be merged into the codebase, 👀 and you will see it in the next version of the app. 
+    
+    The language selection dialog shows the translation progress in a specific language, so let's collaborate to accomplish more! 🎉
+    
+    <img width="300" src="https://user-images.githubusercontent.com/5019152/185754048-31a7aa75-f317-4d1d-a8d6-5cc6abfa7946.png">
+    
+    <img width="300" src="https://user-images.githubusercontent.com/5019152/185754056-d251fa7f-819c-40b2-988c-cd22c8baddb3.png">
+   
+    
+
+    
+