Enhance info related to translation in README

[gturri]

• Update instructions to install the translation files (I just tested with a recent version of Question2Answer and it turns out I had to rename the file but to put it in the plugin directory; which makes sense according to this $include : https://github.com/q2a/question2answer/blob/dev/qa-include/qa-base.php#L1437 )
• Add a link to a pack of French translation I just made so that other French users won't have to bother translating them too