add translation guide

src/languages/README.md

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+# How to translate
+
+1. Fork this repo.
+2. Create a language file. (e.g. zh-TW.js) The filename must be ISO language code: http://www.lingoes.net/en/translator/langcode.htm
+
+3. `npm run update-language-file`
+4. Your language file should be filled in. You can now translate now.
+5. Make a pull request when you have done.
+
+If you do not have programming skills, let me know in Issue section. I will assist you. 😏
+