Add some doc for contributors

docs/translations.markdown

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+Translations
+============
+
+How to translate Kanboard to a new language?
+--------------------------------------------
+
+- Translations are stored inside the directory `app/Locales`
+- There is sub-directory for each language, by example for the French we have `fr_FR`, Italian `it_IT` etc...
+- A translation is a PHP file that return an Array with a key-value pairs
+- The key is the original text in english and the value is the translation for the corresponding language
+- **French translations are always up to date**
+- Always use the last version (branch master)
+
+### Create a new translation:
+
+1. Make a new directory: `app/Locales/xx_XX` by example `app/Locales/fr_CA` for French Canadian
+2. Create a new file for the translation: `app/Locales/xx_XX/translations.php`
+3. Use the content of the French locales and replace the values
+4. Inside the file `app/Model/Config.php`, add a new entry for your translation inside the function `getLanguages()`
+5. Check with your local installation of Kanboard if everything is ok
+6. Send a pull-request with Github
+
+How to update an existing translation?
+--------------------------------------
+
+1. Open the translation file `app/Locales/xx_XX/translations.php`
+2. Missing translations are commented with `//` and the values are empty, just fill blank and remove comments
+3. Check with your local installation of Kanboard and send a pull-request