Update for Translation guidelines

[slasherfun]

Hello,

https://www.ganttproject.biz/support#devs links to guidelines for translators, which needs a few updates:

• "How to translate" section: the file to choose is currently "GanttProject 3.0", instead of "Gp 2.6 Brno"
• "How to test" section: the "right place" is now plugins/base/biz.ganttproject.app.localization/translations
• "How to test" section: when downloading from Crowdin, you now automatically get a file named i18n.<language_code>[_<country_code>].properties, there's no need to rename the downloaded file anymore
• "Editing resource bundles in offline editor" section: it seems that the resource bundles are now written in plain text rather ASCII representation of Unicode symbols, so that whole section may be removed