add contributing.md

CONTRIBUTING.md

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+## Source Code
+
+### Backend
+
+TODO
+
+### Frontend
+
+TODO
+
+## Add translation
+
+### Translation file
+
+All translation files are in `packages/*/locale` directory. Those files are in yaml format. You can add a new language by creating a new file with the language id as the file name. For example, if you want to add a new language `ko_KR`, you can create a new file or edit the existing `packages/*/locale/ko_KR.yml` (not `ko-KR.yaml`), in the following format (all comments should be removed): 
+
+```yaml
+# packages/*/locale/ko_KR.yml
+__id: ko_KR # If you are creating a new language, besure to add this language meta
+__langname: 한국어 # Language name in the language itself
+key1: value1 # translations
+key2: value2
+```
+
+Please sort all keys in alphabetical order, ascending. If you are using vscode, you can select all file by Ctrl+A, then use Ctrl-Shift-P to open command palette, and use `sortLinesAscending` action.
+
+Translation for each file should be put into each module seperately. Do not mix them together.
+
+Currently we only accept translations from native speaker. **DO NOT USE MACHINE TRANSLATION.**
+
+### Translation string
+
+Substitution is supported in translation strings (both array-style and object-style).
+Key and value should be quoted only when needed.
+
+```yaml
+Welcome to {0}, {1}: '{0} 오신 것을 환영합니다, {1}' # array style
+Welcome to {place}, {name}: '{place} 오신 것을 환영합니다, {name}' # object style
+```
+
+Those translation strings will be used in the following format:
+
+```js
+// For backend (in template):
+_('Welcome to {0}, {1}').format(place, name)
+_('Welcome to {place}, {name}').format({ name, place })
+
+// For backend (in code):
+i18n('Welcome to {0}, {1}').format(place, name);
+i18n('Welcome to {place}, {name}').format({ place, name });
+
+// For frontend:
+substitute(i18n('Welcome to {0}, {1}'), [place, name]);
+substitute(i18n('Welcome to {place}, {name}'), { place, name });
+```
+
+### Fallback logic
+
+Assume user's language is `ko_KR`. If there is no `ko_KR.yml` file, it will fallback to `ko.yml`. If there is no `ko.yml` file, it will fallback to `en.yml`. If there is no `en.yml` file, it will fallback to the key.