Add link to crowdin for translators

Author: wilsonge

README.md

@@ -70,6 +70,7 @@ Do you want to improve Joomla?
 * Get Involved: Joomla! is community developed software. [Join the community](https://volunteers.joomla.org).
 * Documentation for [Developers](https://docs.joomla.org/Special:MyLanguage/Portal:Developers).
 * Documentation for [Web designers](https://docs.joomla.org/Special:MyLanguage/Web_designers).
+* Provide a translation for Joomla: [Joomla Crowdin Project](https://joomla.crowdin.com/cms)
 
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